2. The Bacterial Cell Architecture and Shape Is Altered by Antibiotic Resistance
Antibiotics frequently promote (or select for) a few phenotypic variants in the population (known as “persisters”) that tolerate antibiotic action at the expense of changing the cellular architecture, resulting in altered cell shapes. In
E. coli tolerating ampicillin exposure, the persisters are frequently smaller and the cells are swollen, altering physical molecular interactions by changing elasticity or surface-to-volume ratio
[22]. In
Staphylococcus aureus, antibiotic persisters are usually recognized as “small colony variants”, frequently changing the envelope (thicker cell walls) and, correspondingly, a denser granulation in the peripheral cytoplasm, or have branched or multiple cross walls, which are sometimes defective
[23]. In addition, exposure to bacteriostatic agents, such as macrolides and chloramphenicol, results in small colony variants
[24]. Given that it also occurs in pH-driven transitions, bacterial dormancy or persistence could possibly be due to a transition of the cytoplasm from a fluid to a gel-solid-like state
[25][26].
Changes in cell shape induced by antibiotic action have real consequences for the molecular mobility inside the cells (and consequently metabolic activity), which has been tested by using the intracellular diffusion of green fluorescent protein (GFP) in the presence of antibiotics
[27][28][29]. The hyperexpression of resistance mechanisms might also affect bacterial architecture. In general, protein hyperexpression leads to the formation of small colony variants and reduces bacterial growth, as was signaled in recombinant strains modified to hyperproduce (for instance using strong promoters) proteins for chemical or pharma industries. To address this problem, the shape of the bacteria should be modified (through morphology engineering)
[30]. High levels of TEM-1 beta-lactamase expression results in sequestration of the mature excreted polypeptide in insoluble protein aggregates (inclusion bodies) located within the periplasmic space. Given that correct protein folding is altered in these aggregates, the function is lost
[31]. The constitutive hyperexpression of the AmpC-type beta-lactamase might also have a deleterious effect on cellular fitness
[32][33]. Studies on
Salmonella, the only
Enterobacteriaceae that does not possess AmpC, have shown that the acquisition of exogenous AmpC causes changes in cellular morphology, with longer cells indicating an effect on septation, and produces small changes in the structure of the peptidoglycan. These effects fully abolish
Salmonella Typhimurium viability, unless
ampC expression is under control of its regulator, AmpR
[32]. A similar effect of a severe fitness cost occurs in
Pseudomonas aeruginosa [33]. The metallo-carbapenemase NDM-1 is related with lipoproteins of the OM and can be packaged into the periplasmic space by physical interactions with the OM layer, determining “envelope stress” and facilitating its extrusion inside membrane vesicles
[34]. MDR efflux pump hyperexpression should also physically disturb the anatomical and functional structure of the cell envelope, with consequent effects of an altered cell physiology. However, this structural disturbance is still an underexplored area. The fact that particular
Stenotrophomonas maltophilia resistant mutants present a reduced cell size and a lower plate efficiency might shed light the direction of future studies
[35].
Nevertheless, efflux pump overexpression modifies the activity of other unrelated cell machineries, thereby providing an example of structural epistasis. This is the case for MexEF-OprN in the
Pseudomonas aeruginosa, of which the overexpression increases oxygen respiration, modifies the intracellular pH and triggers the activation of the nitrate respiratory pathway
[36]. Notably MexEF-OprN overexpression is associated with a reduction in type III secretion
[37], and type III secretion requires significant activity of the proton motive force
[38]. Whether MexEF-OprN overexpression inactivates type III secretion by modifying the proton motive force, thereby impeding the simultaneous induction of these two energetically costly cell machineries, and if the reduction in fitness is also influenced by less efficient basic metabolic pathways (as energy in invested in overexpression of the secreted protein), are questions that remain unanswered.
The conjugative process of plasmids (which eventually encode antibiotic resistance) might produce (probably small) alterations in the bacterial architecture of the donor cell, because of the relaxosome protein complex (docked to the Type IV secretion system) and the entire conjugative apparatus, forming an envelope structure bridging the IM and the OM
[39]. In the donor and recipient cells, the exit and the entry of the ssDNA plasmid triggers the local recruitment of Ssb (single-strand binding protein) molecules and the formation of membrane conjugative foci, which are apparently located at specific membrane positions, possibly related to the density and stability of the outer membrane protein OmpA, a beta-barrel porin collaborating in the process
[40]. Eventually, mobile genetic elements (preferentially small plasmids) might contribute to increasing particular gene dosages, and thus gene dosage toxicity is based on abnormal protein abundance levels. Dihydrofolate reductase overexpression in
E. coli causes a metabolic imbalance, reducing bacterial fitness
[41].
3. Cell Architecture and Cell Size Influences Antibiotic Effects
Any change in the cell shape requires an expansion or constriction of the membrane layers (and the periplasm volume), which involves a quantitative change in their molecular components or in the distance between them, and therefore their physical interaction. The concept of structural epistasis is applied here to discuss whether these changes might have functional consequences, including the susceptibility to antimicrobial action
[42]. Quantitative modeling has shown that bacteria might adapt (reduce their susceptibility) to antibiotic challenges by reducing the surface-to-volume (S/V) ratio, and consequently, the antibiotic influx and intracellular concentration. In certain cases, however, increasing this ratio might provide an increase in antibiotic efflux rate, and thereby reduced susceptibility. Moreover, the concentration of membrane-associated antibiotics is reduced
[43]. In fact, most tested antibiotics decrease the S/V ratio. Such a reduction also decreases nutrient uptake, slowing the bacterial metabolism and consequently the antibiotic action
[44]. As expected, the morphological cell response to membrane-targeting and membrane-transport-targeting antibiotics is a reduction in the cell surface area. Other effects of the S/V can be considered secondary to the bacterial adaptation to drug action; for instance, the inhibition of translation by ribosome-targeting antibiotics (as chloramphenicol) is compensated by a higher ribosomal biosynthesis, leading to the predominance of growth versus replication, with the consequences of an increased cell volume
[44].
Membrane changes are not necessarily global. Different bacterial processes are confined to membrane microdomains that are similar to lipid eukaryotic cell lipid rafts, which accumulate multimeric protein complexes favoring their oligomerization. One of these proteins, PBP2a, causes beta-lactam resistance in methicillin-resistant
Staphylococcus aureus (MRSA). Notably, the disruption of these membrane microdomains with available drugs (such as statins, regularly used for treating hypercholesterolemia) interferes with PBP2a oligomerization, resulting in MRSA infections that are treatable with penicillin
[45]. This example shows that structural alterations in a cellular element (the cell membrane) impedes the structural changes (oligomerization) required for the activity of a wide-spread antibiotic resistance gene.
In addition, bacteria readily alter their shape in response to non-antibiotic cues. For instance, during urinary tract infections,
E. coli produces long multi-nucleated filaments in response to a still unknown urine component
[46]. In addition to decreasing the S/V ratio, filamentation has numerous consequences for the bacterial lifestyle. For instance, filamented bacteria are less likely to be attacked by phagocytes
[47], and thanks to the extra body-mass, filamented bacteria might have an improved ability to resist shear forces in the bladder and adhere to the epithelium
[48][49]. Beyond the urinary tract, filamentation allows intracellular bacteria to spread among host cells
[50] and might promote the evolution of antibiotic resistance
[51], which provides an excellent example of how physical changes in cell structure can shape numerous, non-related phenotypes.