Other factors that determine the number of survivors

Apart from the initial inoculum size, the concentration of the antibiotic, the time of incubation and the type of antibiotics influence the number of bacterial survivors following exposure to antibiotics.

Antibacterial activity of antibiotics, in general, is proportional to its concentration. However, at very high concentrations of some antibiotics, this may not be true. Antibiotics are most effective in a narrow range of concentration above the MIC. When the concentration of antibiotics is well above the MIC, a paradoxical effect can be noticed, wherein some bacteria are not killed but are only inhibited from growing. This is well documented with nalidixic acid and also reported with other antibiotics such as β-lactam antibiotics. Paradoxical effect of oxacillin and ciprofloxacin was recently demonstrated by Udekwu et al. (2009). With high inoculum size, the number of survivors after 18 h of antibiotic treatment was more with 100 times the basal MIC (bMIC) than with 5 times the bMIC. This was true for ciprofloxacin also.

Total time of incubation also influence the number of survivors. The importance of this factor was also demonstrated by Udekwu et al. (2009). In certain cases, the number of survivors is high after a short incubation period of 3 h but progressively decreases following 18 h of incubation (ex: vancomycin at 20 times the bMIC against low cell density), whereas in other cases the number of survivors increases following 18 h of incubation (ex: daptomycin at 5 times the bMIC against low cell density), which was not due to the re-growth of mutant population.

The type of antibiotics is another factor in determining the number of survivors. Concentration dependent antibiotics such as aminoglycosides (ex: gentamicin or tobramycin) produce low number of persisters. This group of antibiotics exhibit minimal inoculum effect and paradoxical effect. The lack of persisters following exposure to aminoglycosides, reported in many experiments, is due to the property of the antibiotic only.

The demonstration of persisters following incubation of a single inoculum size of bacterial culture with a single antibiotic concentration for a specific period of time (3-6 h) is questionable. The persisters and their phenotypic shift demonstrated in many experiments may not be a survival strategy of the bacterial population, but rather the property of antibiotics itself which have been known and reported decades back. The dosage regimen of the antibiotics have been optimized only after considering all these factors.

Next- Persisters induced by antibiotics- Old wine in new bottle?

Udekwu et al. (2009). Functional relationship between bacterial cell density and the efficacy of antibiotics. J Antimicrob Chemother 63(4), 745-57.