Bacterial structure and the cell envelope

Bacteria are cells without a membrane-bound nucleus. Their cytoplasm is enclosed by a cytoplasmic membrane, which controls what enters and leaves. Outside that membrane, many bacteria have a cell wall; some also carry additional surface layers or structures. Together, these coverings and attachments shape how a bacterium interacts with its surroundings.

The cytoplasmic membrane is a flexible boundary made mainly of phospholipids and proteins. The phospholipids form a bilayer: their water-attracting heads face the watery environments inside and outside the cell, while their water-repelling tails face inwards. Membrane proteins help move selected substances across the bilayer, receive signals and carry out other essential cell functions. The membrane is not a rigid shell. Its fluidity describes how readily its components move within the bilayer; technically, fluidity is the reciprocal of viscosity. A functioning membrane must remain sufficiently flexible to support transport and other processes.

The cell wall sits outside the cytoplasmic membrane and provides mechanical strength. It helps the cell maintain its shape and resist the tendency to swell and burst in a watery environment. In most bacteria, this strength comes largely from peptidoglycan, a mesh of sugars linked by short peptides. The wall is not a second membrane: it is more porous and does not perform the same selective transport role as the cytoplasmic membrane.

A useful structural distinction is between Gram-positive and Gram-negative bacteria. Gram-positive bacteria have a thick peptidoglycan wall outside the cytoplasmic membrane. Gram-negative bacteria have a thinner peptidoglycan layer in a periplasmic space, between the cytoplasmic membrane and an additional outer membrane. This difference affects both the layout of the envelope and how substances encounter it. The bacterial wall can impede the transport of compounds with molecular weight higher than 600–1000 Da. ^[1]

Thinking in layers helps make sense of bacterial structure. The cytoplasmic membrane is the living cell’s selective boundary; the wall reinforces it; and any outer membrane or coating adds further properties. These layers are related, but their different materials and positions mean they should not be treated as interchangeable.

Bacterial surface structures are not all layers of the envelope. Some are appendages that help cells move, attach or exchange material; others are protective forms or coatings. A bacterium may have several of these features, but their presence and arrangement vary between species and between cells.

A flagellum is a long, slender appendage used by many bacteria for movement. Its exposed filament is connected to the cell by a curved hook and a motor embedded in the envelope. Rotation of the motor turns the filament, pushing or pulling the cell through liquid. Changes in the direction or pattern of movement can help a bacterium respond to chemical conditions: movement towards a favourable chemical or away from a harmful one is called chemotaxis. A flagellum is therefore both a mechanical structure and part of a behavioural response. It should not be confused with a eukaryotic cilium, which has a different internal construction and movement mechanism.

Pili are hair-like protein appendages on the cell surface. Some help bacteria attach to host tissues, other cells or non-living surfaces. This attachment can help a cell remain in place despite fluid movement. Certain pili also bring bacterial cells together so that genetic material can be transferred between them. Some pili can extend and retract, pulling a cell across a surface; this is called twitching motility. Fimbriae are usually shorter, numerous attachment fibres, although everyday usage of “pili” and “fimbriae” can overlap. These structures are appendages, not envelope layers.

Some bacteria can form endospores, dormant structures that help them survive conditions that would damage growing cells. An endospore contains the bacterial genetic material and is surrounded by protective layers. It is a survival form, not a means of reproduction: when conditions become suitable, it can germinate and return to a growing cell. Spore formation is a specialised response found in some bacteria, not a routine stage in every bacterial life cycle.

Capsules and slime layers are forms of an external coating, often grouped under the name glycocalyx. A capsule is relatively organised and closely associated with the cell surface. A slime layer is more diffuse and loosely attached. Both can help bacteria adhere to surfaces and contribute to biofilms, in which cells live within a shared matrix. A coating may also reduce drying and make it harder for host defences to remove or engulf a bacterium. Its exact effects depend on the coating and the setting.

The outer membrane is an additional envelope layer found in Gram-negative bacteria. It lies outside the peptidoglycan wall, with a space containing that wall between the outer membrane and cytoplasmic membrane. The cell therefore has an inner cytoplasmic membrane and an outer membrane, but the two are not identical. The cytoplasmic membrane is the main selective boundary around the cytoplasm; the outer membrane forms an additional barrier between the cell and its environment.

The outer membrane has an unusual composition. Its inner-facing leaflet is mainly phospholipid, while its outer-facing leaflet contains lipopolysaccharide, usually abbreviated to LPS. LPS includes lipid A, a core region and an outward-facing O antigen. These parts contribute to the membrane’s organisation and surface properties. LPS is also important in interactions with host defences: some antimicrobial peptides bind outer-membrane LPS, and this interaction can contribute to bacterial-cell agglutination^[1].

Proteins embedded in the outer membrane help control passage through it. Porins form channels that allow selected small, water-soluble substances to cross, while the membrane’s lipid-rich regions restrict many other compounds. This barrier can reduce access to the cell interior, but it is not an impermeable shield. Substances may pass through channels, interact with the surface or be blocked from reaching the cytoplasmic membrane. The outer membrane and the wall therefore add barriers without replacing the selective transport functions of the inner membrane.

The outer membrane is also a target for some antimicrobial agents. Colistin interacts with LPS and disrupts the outer membrane; effective killing also involves damage to the inner membrane, and bacterial resistance can alter susceptibility. This illustrates why the envelope is best understood as a connected set of layers: disrupting an external barrier and damaging the membrane around the cytoplasm are related but distinct events.

Gram-negative bacteria can release outer membrane vesicles, or OMVs. These are spherical portions of envelope material containing outer-membrane proteins and lipids, together with material in or on the vesicle. OMVs are produced during growth and have proposed roles in virulence, inflammation and responses to envelope stress. They can also interact with external stressors. In experiments, pre-incubating T4 bacteriophage with OMVs was associated with binding and reduced infection. Vesicles can thus affect interactions beyond the cell surface, though their effects depend on the particular vesicle and stressor.

The cell envelope is the set of structures separating the bacterial cytoplasm from the surroundings. Its layers are not the same in every bacterium. In Gram-negative bacteria, the periplasm is the space between the cytoplasmic membrane and the outer membrane. It is not simply an empty gap: it contains the peptidoglycan wall and proteins that help process molecules and maintain the envelope. The outer membrane, including its lipopolysaccharide (LPS), is an additional boundary; the cytoplasmic membrane remains the selectively permeable barrier around the cytoplasm.

Peptidoglycan, also called murein, is a mesh-like polymer outside the cytoplasmic membrane. Its sugar chains are joined by short peptide links, forming a strong but flexible wall. The mesh helps the cell retain its shape and withstand internal osmotic pressure, while still allowing small solutes to pass through its pores. It is not a sealed container: selective transport across the envelope depends on membrane proteins, and the wall’s main role is structural support. Damage to the mesh can weaken the cell and make it vulnerable to bursting, especially when water moves into the cytoplasm.

Gram-positive bacteria have a thick murein wall, often associated with teichoic acids. They do not have the Gram-negative arrangement of an outer membrane and periplasm. Gram-negative bacteria, by contrast, have peptidoglycan within the periplasm, between the inner and outer membranes. These differences matter when considering how substances reach the cytoplasmic membrane: the outer layers can alter access, but a thick wall should not automatically be treated as an impermeable shield.

A distinctive example is the group of Gram-positive bacteria known as Mycolata, which includes Rhodococcus, Mycobacterium, Nocardia, Corynebacterium, Gordonia, Dietzia, Skermania and Tsukamurella. Their envelope contains abundant arabinogalactan linked to branched-chain mycolic acids. This organised complex is associated with a hydrophobic cell surface and low permeability, helping explain the group’s notable resistance to drugs and hydrophobic toxic compounds. This is a special envelope arrangement, not a general property of every Gram-positive bacterium.

The wall can also be recognised by molecules that bind to it. Some cell-agglutinating antimicrobial peptides interact directly with Gram-positive peptidoglycan or Gram-negative outer-membrane LPS. Rather than making pores, they can cause bacterial cells to clump; those clumps can then be removed by phagocytosis. Thanatin is an example: it is an inducible antimicrobial peptide of 21 residues. It binds LPS or peptidoglycan and can produce cell agglutination. These examples show that envelope components are not merely passive packaging: they can determine how bacteria interact with antimicrobial molecules. ^[2]

The cytoplasmic membrane is a dynamic boundary around the cytoplasm. Its phospholipid molecules form a bilayer: their water-attracting heads face the watery environments inside and outside the cell, while their water-repelling tails face one another. This arrangement creates a barrier to many molecules. Proteins within the bilayer provide selective routes across it, including pumps and channels that move particular substances into or out of the cell. The membrane is therefore both a physical boundary and a working interface for transport and cellular activity.

In most bacterial strains, phosphatidylethanolamine (PE) makes up 75%, phosphatidylglycerol (PG) 15–20%, and cardiolipin (CL) 5–10% of the membrane phospholipids. These are common proportions, not a rule that applies identically to every species. Phosphatidylcholine (PC), familiar in mammalian membranes, occurs in only a few bacterial species. Examples include Rhodopseudomonas sphaeroides and Pseudomonas stutzeri. The range of lipid headgroups affects the membrane’s physical properties and helps explain why bacterial membranes are not all chemically identical. ^[1]

Making membrane lipids requires substantial resources. In the account of phospholipid biosynthesis, producing a mole of phospholipid from acetyl-CoA and sn-glycerol-3-phosphate requires 32 mol of ATP. Phospholipids also constitute 10% of the dry weight of the cell. These figures illustrate that the membrane is a significant investment in cell material and energy, rather than a thin, expendable wrapper. ^[1]

The bilayer must remain fluid enough for its components to move, while retaining enough stability to act as a barrier. Bacteria can adjust fatty-acid composition, fatty-acid chain length and phospholipid composition as conditions change. Exposure to hydrophobic compounds can disturb membrane fluidity; changing the lipid mixture can help counter that effect and preserve a workable balance between bilayer-forming and non-bilayer-forming lipids. PE is especially abundant and contributes lateral pressure within the bilayer. Its shape favours non-bilayer arrangements, which are relevant to membrane fusion, cell division and the movement of membrane components. The balance among lipid types can therefore affect how the membrane behaves, not just what it is made of. ^[1]

Membrane proteins are affected by the surrounding lipids. The membrane contains proteins involved in solute transport and electron transport, and their activity depends in part on membrane properties such as fluidity, stability and thickness. Changes to the lipid environment can alter protein activity; for example, polycyclic aromatic hydrocarbons have been associated with decreased cytochrome c activity. Thus, membrane composition can influence both the passage of substances and the operation of proteins embedded in the bilayer.

Some antimicrobial peptides act directly on the cytoplasmic membrane. They can kill bacteria by forming pores, with proposed mechanisms including the barrel-stave model and toroidal-pore model, or by detergent-like disruption. This differs from cell-agglutinating peptides, which bind envelope components without permeabilising the membrane. The distinction links membrane structure to mechanism: a peptide may damage the bilayer itself, or act at the wall or outer membrane instead.

A bacterial envelope is the set of structures around the cytoplasm. It protects the cell, controls what can enter or leave, and provides surfaces that interact with the host and the environment. The cytoplasmic membrane is present in both Gram-positive and Gram-negative bacteria. Outside it, the main structural difference is the arrangement of the cell wall and the presence or absence of an outer membrane.

Gram-positive bacteria have a thick peptidoglycan wall outside the cytoplasmic membrane. Peptidoglycan is a strong mesh made from linked sugar chains and short peptides; it helps the cell resist bursting when water moves into it. Teichoic acids and lipoteichoic acids are associated with this wall and can affect its surface properties. Some species also have capsules or other outer layers. Cell-wall polymers can provide binding sites for surface proteins: pyruvylated polymers in Bacillus anthracis and Paenibacillus alvei are described as high-affinity ligands for such attachment.

Gram-negative bacteria have a thinner peptidoglycan layer in the space between the cytoplasmic membrane and an outer membrane. The outer membrane is a selective barrier. It contains lipopolysaccharide (LPS) on its external face, alongside proteins that help control passage through the envelope. LPS is not just a structural coating: its composition influences surface properties. Changes in LPS composition have been associated with changes in hydrophobicity, adhesion and permeability. The outer membrane can also make entry more difficult for some antimicrobial molecules, while channels and transport systems regulate access to the cell.

These structures explain the different results of a Gram stain. The stain forms a coloured complex inside bacterial cells. In Gram-positive cells, the thick peptidoglycan wall retains that complex during decolourisation, so the cells appear purple. In Gram-negative cells, decolourisation disrupts the outer membrane and the thinner wall does not retain the complex as effectively; the counterstain then makes the cells appear pink or red. This is a useful structural clue, not a complete identification: staining appearance does not by itself establish the species or predict which antibiotics will work.

Envelope differences matter clinically because they shape access, recognition and resistance. A drug that reaches exposed peptidoglycan may have a different opportunity to act in Gram-positive and Gram-negative cells. In Gram-negative bacteria, the outer membrane can limit entry, and enzymes in the space around the inner membrane may inactivate some drugs before they reach their targets. LPS is also important in host responses: its lipid component can trigger inflammation when released or encountered during infection. The exact consequences depend on the organism, the site of infection and the host response, so the Gram reaction alone is not a treatment decision. ^[4]

The envelope can adapt under stress. In Gram-negative bacteria, outer-membrane vesicles (OMVs) have been proposed as an intrinsic defence that can modulate responses to antimicrobial peptides and other stresses affecting the outer membrane. Vesiculation may provide short-term defence against low-dose cell-wall stressors and contribute to longer-term protection in hostile environments. OMVs can also package bacterial factors for delivery to host cells, including across mucus barriers and without direct contact between the bacterium and host cell. These findings help explain why the envelope is both a barrier and an active interface, but they do not make vesicle production a routine clinical diagnostic marker.

In a clinical laboratory, Gram staining gives a rapid view of bacterial shape, grouping and broad envelope type. A specimen is spread thinly on a slide, fixed, stained, decolourised and counterstained. The result can help guide interpretation of a specimen and the choice of culture conditions while a more specific identification is pending. The stain is most informative when the sample is suitable and the observed organisms can be distinguished from host cells, debris or contaminants.

Microscopy should be interpreted alongside the clinical source and specimen quality. Shape and arrangement can narrow the possibilities, but many unrelated species look similar under a light microscope. A negative stain does not rule out infection: organisms may be sparse, unevenly distributed, damaged by treatment or difficult to visualise. Some bacteria also stain poorly with the standard method because of unusual envelope features. In those cases, a different stain, specialised culture method or molecular assay may be needed.

Culture allows organisms to grow so that they can be identified and tested. Colony appearance and growth characteristics provide clues, while biochemical tests, mass spectrometry or molecular methods can refine identification. The Gram reaction helps organise this work because the envelope influences growth requirements and the choice of follow-up tests. However, identification should rest on the combined evidence rather than on colour or morphology alone.

Antimicrobial susceptibility testing examines whether an isolate is inhibited by particular drugs under defined laboratory conditions. It is needed because envelope class is only a broad predictor: bacteria within the same Gram group can differ in drug targets, drug entry, drug-inactivating enzymes and resistance mechanisms. Gram-negative outer-membrane changes can affect access to the cell, and antimicrobial-peptide resistance can involve lipid modifications that alter outer-membrane charge. A laboratory susceptibility result therefore adds information that staining cannot provide.

Research laboratories also investigate envelope structure directly. Imaging methods can show the layers and vesicles, while chemical and biophysical methods examine LPS, membrane properties and interactions with antimicrobial molecules. Nuclear magnetic resonance (NMR), for example, can be used to study molecular structure and contacts in solution. A reported thanatin structure was determined in aqueous solution at 25 °C, following an earlier solution structure reported at 5 °C. Its structural ensemble was based on 140 NOE-driven distance constraints. This is a research example of how molecular interactions with envelope components can be studied, rather than a routine test for diagnosing bacterial infection. ^[2]

A further example links molecular structure to LPS binding. In an LPS-associated thanatin dimer, reported cationicity was 12.8 compared with 7 for the monomer, and exposed hydrophobic surface was 2203 Å2 compared with 1603 Å2. Such measurements help investigate how antimicrobial molecules interact with the Gram-negative outer membrane. In clinical practice, the practical sequence remains to interpret the specimen, establish an organism’s identity and assess its susceptibility where indicated; the envelope provides the framework for understanding those results, not a substitute for them. ^[2]

The cell envelope is a useful drug target because it is both a protective boundary and a structure the bacterium needs to maintain. In Gram-negative bacteria, a thin peptidoglycan layer is linked to an outer membrane whose predominant added component is lipopolysaccharide (LPS). LPS combines a sugar chain with a lipid region; its closely packed fatty acids make the outer membrane poorly permeable to hydrophobic compounds. The sugar chain also contributes to resistance: Escherichia coli mutants unable to make these chains were more sensitive to hydrophobic antibiotics, detergents and other drugs. Thus, a drug’s activity depends not only on its target but also on whether it can cross the envelope and reach it.

The outer membrane is not an impermeable sheet. Porins are water-filled protein channels that allow nutrients and water to enter; some selectively transport particular molecules, while others allow less selective diffusion. Changes in porins can alter access for hydrophilic antibiotics and contribute to resistance. This explains why resistance can arise without changing a drug’s binding site: reduced entry may leave too little drug inside the bacterium to act effectively. Conversely, a damaged or altered envelope may let compounds enter more readily. In practice, the barrier, the channels and the drug target should be understood as connected parts of the same problem.

Some treatments act on the peptidoglycan wall, while others attack or disrupt the outer membrane. β-lactams are among the peptidoglycan-targeting drugs discussed in relation to mycobacteria; β-lactam/β-lactamase inhibitor combinations are described as a potential route for treating mycobacterial infections. The distinction matters: weakening wall construction and disrupting an outer membrane are different strategies, and a drug’s usefulness depends on the envelope of the organism being treated.

Colistin is a polymyxin antibiotic that damages the bacterial envelope. Murepavadin, an LPS transport inhibitor, potentiated colistin’s activity at a colistin concentration of 1 µg ml−1. In the reported experiments, the combination increased colistin’s ability to damage the cytoplasmic membrane, leading to bacterial lysis and cell death. This illustrates how blocking envelope assembly can make a membrane-active antibiotic more effective. It is a mechanistic finding, not by itself a treatment recommendation. ^[4]

The animal experiment used intranasal treatment in a mouse lung-infection model. The colistin dose was 5 mg kg−1 and the murepavadin dose was 0.25 mg kg−1. These doses were chosen to mimic treatment of human lung infections, and the delivery route was described as similar to clinical delivery. After 3 hr, treatment with either drug alone had very little effect on bacterial load compared with no treatment. The result reported here does not establish the benefit, safety or appropriate dose of the combination in people. Such distinctions are essential when translating a cell-envelope mechanism or animal experiment into clinical care. ^[4]

Cell-envelope biology varies across bacterial groups, so a treatment that acts at one envelope feature cannot be assumed to work equally well against every organism. Gram-negative bacteria have an LPS-containing outer membrane, with porins that influence the entry of small molecules. Mycobacteria present a distinct treatment challenge: drug-resistant Mycobacterium tuberculosis strains are described as a major global threat that current drug-discovery efforts are not adequately meeting. The discussion of peptidoglycan-targeting drugs in mycobacteria points to β-lactam/β-lactamase inhibitor combinations and phage endolysins as possible avenues for future treatment, rather than established universal regimens.

Resistance can also differ between strains and can change under drug pressure. In experiments, polymyxin B treatment induced outer-membrane vesicle production in enterotoxigenic E. coli (ETEC), with production nearly 7-fold higher than in untreated cultures. The reported controls found no significant cell lysis, supporting the interpretation that the vesicles were produced as a response rather than simply released from cells that had broken apart. Vesicles can bind or absorb stressors that act at the cell surface; their release may therefore help protect the producing bacterium and nearby bacteria. This is a community-level defence, not just a change in an isolated cell. ^[3]

The proposed model is that outer-membrane stress can trigger vesiculation in some bacteria while other cells die. Vesicles may remove some of the stressor from the immediate cell surface and increase the local pool able to bind further stressors. A period of increased vesicle release may also coincide with changes to LPS that reduce binding by antimicrobial peptides. These mechanisms help explain why an antimicrobial challenge can select for or support bacterial defences rather than simply eliminating every exposed cell. They also point to possible ways that outer-membrane-targeting treatments could be undermined; the findings do not, however, specify a clinical test or treatment adjustment.

For patients in special clinical circumstances, envelope biology alone cannot determine a suitable regimen. Age, pregnancy, kidney function, allergy history, infection site and other clinical factors may affect prescribing, but no subgroup-specific doses or treatment criteria are provided here. The mouse doses described above should therefore not be used as human dosing instructions. Clinical decisions need to account for the identified organism and its resistance pattern as well as the patient’s circumstances.

At a global level, resistant tuberculosis makes the search for new envelope-directed treatments especially important. More broadly, bacterial cell-wall biology remains an area of active investigation for developing compounds against antibiotic-resistant infections. These are research priorities, not evidence that a particular candidate is already effective or recommended for routine care. A sound approach is to connect the drug’s target to the organism’s envelope, distinguish laboratory and animal findings from clinical evidence, and avoid treating a promising mechanism as a proven regimen.

The bacterial cell envelope is the boundary between the cytoplasm and the external environment. It is not a single structure: the cytoplasmic membrane controls exchange with the cell, while the wall and any outer layers contribute mechanical support and protection. The exact arrangement differs between bacterial groups. Gram-positive bacteria have a thick peptidoglycan wall outside the cytoplasmic membrane. Gram-negative bacteria have a thinner peptidoglycan layer in a periplasmic space and an additional outer membrane.

Membranes are sheet-like compartment boundaries, with thickness between 6 nm and 10 nm (60–100 Å). A phospholipid bilayer forms the basic barrier: water-facing heads border aqueous environments, while fatty-acid chains face inwards. Embedded proteins carry out functions such as transport and signalling. The membrane is therefore not just a wrapper; its structure helps determine what can cross it and how the cell responds to its surroundings. ^[1]

Cardiolipin is a minor membrane component found in many bacterial strains. Together with phosphatidylglycerol, it represents the most abundant anionic lipid component in bacterial membranes. Each cardiolipin molecule contains four fatty acid chains and has one negative charge per headgroup. It may bind a wide range of unrelated proteins. Cardiolipin synthase acts in the cytoplasmic membrane and transfers phosphatidyl groups between phosphatidylglycerol molecules; the enzyme is also known as phospholipase D. ^[1]

A key feature of the Gram-negative outer membrane is lipopolysaccharide (LPS). LPS is made in the cytoplasm, inserted into the inner leaflet of the cytoplasmic membrane, flipped to the outer leaflet by MsbA, then transported to the outer membrane by the Lpt machinery. This route matters because LPS is not confined to the cell surface while it is being made and moved. The location of LPS, and the membrane containing it, can affect how an antibiotic interacts with the cell.

A common mistake is to treat the cell envelope as if it were a single barrier. The outer membrane and cytoplasmic membrane are distinct structures, with different compositions and roles. In Gram-negative bacteria, the outer membrane contains abundant LPS, while the cytoplasmic membrane contains much less LPS. An observation about damage to one membrane does not automatically tell us what has happened to the other.

This distinction is important for polymyxins such as colistin. Their binding to LPS at the bacterial surface disrupts the outer membrane, but this alone does not explain bacterial killing. The proposed idea that polymyxins cross the outer membrane through “self-directed uptake” remains a hypothesis, not a demonstrated mechanism. Permeabilisation of the cytoplasmic membrane is required for lysis and killing, although the precise mechanism of colistin’s action there has been unclear.

Do not equate outer-membrane damage with successful killing in resistant bacteria. Colistin can damage the outer membrane of bacteria carrying LPS modifications associated with resistance, yet fail to kill or lyse them. This supports the idea that those modifications may protect the cytoplasmic membrane, even when outer-membrane damage still occurs. The envelope must be considered as a sequence of barriers and targets, not as one all-or-nothing shield.

Timing also matters. Colistin caused outer-membrane damage within minutes of exposure, whereas cytoplasmic-membrane disruption took much longer. Even in spheroplasts lacking an outer membrane, cytoplasmic-membrane permeabilisation took more than 2 hr. Rapid surface damage therefore cannot be used as a substitute for evidence of injury to the inner barrier. ^[4]

Finally, do not assume that an immediate defensive effect must protect every bacterium. Outer-membrane vesicles (OMVs) can contribute to bacterial defence, but the described ETEC experiment found incomplete protection: purified OMVs gave 50% protection. The authors note that an immediate innate defence may give bacteria an advantage without protecting the entire population. A partial effect can still be biologically meaningful. ^[3]

Envelope structure helps clinicians and microbiologists interpret how bacteria may respond to antimicrobial agents. The general reasoning is to identify the target layer, then ask whether the observed effect is limited to that layer or extends to the cytoplasmic membrane. This distinction is particularly relevant when resistance is present: apparent outer-membrane disruption does not, by itself, establish that bacteria have been killed.

Colistin is described as an antibiotic of last resort, but its efficacy is poor and resistance is a growing problem. The relevant mechanism is not simply that colistin binds surface LPS. Evidence indicates that colistin can exert bactericidal activity by targeting LPS in the cytoplasmic membrane. This links envelope structure to treatment response: where LPS is located, and whether the inner membrane is vulnerable, may matter as much as surface binding. ^[4]

The described mobile colistin-resistance genes encode phosphoethanolamine transferases that modify the lipid A component of LPS as it travels through the cytoplasmic membrane. The passages describe 10 such gene variants. Resistance can also arise through mutations affecting regulatory systems, including PhoPQ, PmrAB and BasRS. These mechanisms point to the importance of studying LPS during its movement through the envelope, rather than considering only the mature outer membrane. ^[4]

This mechanism has also informed experimental approaches. In a study, inhibiting LPS transport with murepavadin caused LPS to accumulate in the cytoplasmic membrane of Pseudomonas aeruginosa. The bacteria became more susceptible to colistin in vitro, and treatment efficacy improved in vivo. This is a mechanistic finding, not a general treatment instruction. Colistin is currently given by inhalation for some lung infections in people with cystic fibrosis, though it is not always successful; murepavadin was described as approaching clinical trials for inhaled treatment. If those trials are successful, using the drugs together may be possible.