On the development of low level resistance in gram-positive and gram-negative bacteria after antibiotic exposure

• Resistance has been found to develop and spread via several mechanisms against virtually all clinically available antimicrobial agents. Increasing use of antibiotics has been shown to correlate to increasing incidence of bacterial resistance. Hypothetically, the resistant variants appearing in antibiotic-exposed gram-positive and gram-negative bacteria account for a substantial amount of the development of low-level resistance seen during anti-microbial therapy.The aim of the present study was to delineate the occurrence of resistant variants and the selection of variants after exposure to subinhibitory concentrations of antibiotics (1/2 MIC). Experiments were designed to scrutinize aminoglycosides, quinolones, carbapenems and glycopeptides and their effects on relevant organisms, e.g. Pseudomonas aeruginosa, Escherichia coli and enterococcal and staphylococcal species.Resistant variants occurred in unexposed strains in frequencies of I0-4 or below at a concentration of lxMIC of all the antibiotics tested.In strains of staphylococci exposed to amikacin, gentamicin, netilmicin, tobramycin or vancomycin, similar frequencies of variants resistant to the aminoglycosides grew at concentrations of up to 16 x the initial MIC value. In contrast, there was no selective growth of variants at concentrations above lxMIC of the glycopeptide vancomycin. Neither was any growth of resistant variants in strains of enterococci noted after exposure to either teicoplanin or vancomycin. However, cultures of staphylococci exposed to teicoplanin regrew, giving mean frequencies of lQ-5-JQ-3 for resistant variants at concentrations of up to 8 x MIC.In strains of P. aeruginosa, exposure to ciprofloxacin and ofloxacin resulted in similar frequencies of resistant/cross-resistant variants at concentrations up to 32 x MIC. Likewise, regrowth frequencies of imipenem- and meropenem-resistant/cross-resistant variants were similar after exposure to any of the carbapenems and, notably, this was also true for the strains, cross-resistant to carbapenems after quinolone exposure.Considering the MIC-limits after quinolone or carbapenem exposure fewer strains exhibited a shift in susceptibility towards resistance, to the quinolone and carbapenem with the lowest initial MIC value.Similar frequencies of aminoglycoside-resistant variants were found at concentrations of up to 16xMIC for strains of P. aeruginosa and E. coli, although in the former species the initial MIC values and the concentrations at which these variants grew were lower for tobramycin than for the other aminoglycosides. Incubation with the aminoglycosides, atconcentrations corresponding to their respective MIC-limits, resulted in regrowth of 15% of the strains incubated with tobramycin in comparison to 80-90% regrowth after incubation to the other aminoglycosides.The risk of development of drug resistance is thus dependent on the initial MIC value and the factor by which this concentration is exceeded by the actual concentration at which resistant variants are capable of re growth after antibiotic exposure.A comparison of MIC values obtained before and after antibiotic exposure, and whether or not the MIC-limits are reached, might serve as an indicator of the degree to which different bacterial species are apt to develop resistance to a certain antibiotic.

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