Gram-positive bacteria are primed for surviving lethal doses of antibiotics and chemical stress

Antibiotic resistance kills millions worldwide yearly. However, a major contributor to recurrent infections lies in a small fraction of bacterial cells, known as persisters. These cells are not inherently antibiotic-resistant, yet they lead to increased antibiotic usage, raising the risk of developing resistant progenies. In a bacterial population, individual cells exhibit considerable fluctuations in their gene expression levels despite being cultivated under identical, stable conditions. This variability in cell-to-cell characteristics (phenotypic diversity) within an isogenic population enables persister cells to withstand antibiotic exposure by entering a non-dividing state. We recently showed the existence of "primed cells" in E. coli. Primed cells are dividing cells prepared for antibiotic stress before encountering it and are more prone to form persisters. They also pass their "prepared state" down for several generations through epigenetic memory. Here, we show that primed cells are common among distant bacterial lineages, allowing for survival against antibiotics and other chemical stress, and form in different growth phases. They are also responsible for increased persister levels in transition and stationary phases compared to the log phase. We tested and showed that the Gram-positive bacterium Bacillus megaterium, evolutionarily very distant from E. coli, forms primed cells and has a transient epigenetic memory that is maintained for 7 generations or more. We showed this using ciprofloxacin and the non-antibiotic chemical stress fluoride. It is well established that persister levels are higher in the stationary phase than in the log phase, and B. megaterium persisters levels are nearly identical from the early to late-log phase but are ~2-fold and ~4-fold higher in the transition and stationary phase, respectively. It was previously proposed that there are two distinct types of persisters: Type II forms in the log phase, while Type I forms in the stationary phase. However, we show that primed cells lead to increased persisters in the transition and stationary phase and found no evidence of Type I or II persisters with distant phenotypes. Overall, we have provided substantial evidence of the importance of primed cells and their transitory epigenetic memories to surviving stress.

Potential antivirulence activity of sub-inhibitory concentrations of ciprofloxacin against Proteus mirabilis isolates: an in-vitro and in-vivo study

BACKGROUND

Proteus mirabilis is a significant nosocomial pathogen that is frequently associated with a wide range of infections, necessitating heightened attention to mitigate potential health risks. Hence, this study was performed to investigate the impact of sub-minimum inhibitory concentrations (MICs) of ciprofloxacin (CIP) on Proteus mirabilis clinical isolates.

METHODS

The sub-MICs of CIP were selected using the growth curve approach. The untreated and treated isolates with sub-MICs of CIP were assessed for their biofilm development, motilities on agar, and other virulence factors. The cell morphology of untreated and treated isolates with sub-MIC of CIP was explored using electron microscope. Moreover, the expression levels of the virulence genes in isolates were measured using quantitative real-time PCR.

RESULTS

Data revealed that sub-MICs of CIP significantly (p < 0.05), in a concentration-dependent manner, inhibited biofilm formation and other virulence factors in the selected isolates. Electron microscope analysis showed cell enlargement and various abnormalities in the cell wall and membrane integrity.

CONCLUSION

Sub-MICs of CIP exhibited inhibition of virulence and alterations in morphological integrity against P. mirabilis isolates.

In vitro effects of sub-inhibitory concentrations of amoxicillin on physiological responses and virulence determinants in a commensal strain of Escherichia coli

AIMS

The goal was to study the effects of sub-minimum inhibitory concentrations (sub-MICs) of amoxicillin (AMX) on various physiological responses and virulence determinants in a commensal strain of Escherichia coli.

MATERIALS AND RESULTS

The commensal strain was passaged under various sub-MICs of AMX and its effect on bacterial growth, motility, biofilm formation, expression of outer membrane proteins (OMPs) and cell adhesion was analysed. Bacterial growth was diminished at 1/2 and 1/4 MICs of AMX with significant reduction in growth rate. Using crystal violet (CV) assays and quantification of surface polysaccharides we observed strong biofilm formation, together with reduced swimming motility in E. coli at 1/2 MIC of AMX. Differential OMP expression upon AMX sub-MIC exposure coincided with enhanced cell adhesion to HT-29 cells in vitro. The results demonstrated that sub-MICs of AMX can stimulate unpredictable changes in commensal bacterial strains which can be a potent source for the propagation of antibiotic resistance.

CONCLUSIONS

The study reports that AMX at 1/2 MIC significantly compromised bacterial growth and swimming motility, alongside inducing biofilm formation. This was also accompanied by upregulation of a single OMP which subsequently increased cell adhesion capabilities in E. coli at 1/2 MIC, thereby enhancing its colonization and survival abilities within the gut microsphere.

SIGNIFICANCE AND IMPACT OF THE STUDY

For the first time, the effects of AMX sub-MICs on a commensal E. coli strain were described. The results corroborate on how antibiotics can act as stimulatory molecules and determine the pathogenicity of commensal bacteria in vivo that can disseminate resistance to other intestinal pathogens or microbes.