Antibiotic resistance among human pathogens now occurs in almost every bacterial species for which antibiotic therapies exist (15
). In the case of Streptococcus pneumoniae
, resistance to penicillin and erythromycin has become so widespread that clinicians have started to use the fluoroquinolones for therapy. As a result, drug resistance is now emerging against the quinolones (4
). More active fluoroquinolones are becoming available, but new treatment strategies must accompany use of those agents to halt the selection of resistant mutants before the entire quinolone class of drugs becomes ineffective.
It has been suggested that if bacterial cells must attain two concurrent resistance mutations for growth in the presence of a quinolone, then few mutants would be selectively amplified because double mutations should rarely occur (19
). For example, bacterial populations may reach 1010
cells in human infections (1
), but at a mutation frequency of 10−7
, more than 1014
) would be required to detect two concurrent fluoroquinolone-resistant mutations. When we examined the effect of fluoroquinolone concentration on the selection of resistant mutants of Mycobacterium bovis
BCG and Staphylococcus aureus
, we found a concentration with each organism at which no mutant was recovered when more than 1010
cells were applied to agar plates (5
). This drug concentration, which we designated as the mutant prevention concentration (MPC), would require a bacterial cell to develop more than one resistance mutation for growth. Thus at concentrations above the MPC, a bacterial population size greater than that normally present during infection would be necessary to observe outgrowth of a resistant mutant. Since fluoroquinolone structure affects the value of the MPC (5
), it appeared that the MPC might serve as a simple measure of antibiotic potency that incorporates the ability of a compound to restrict selection of resistant mutants (28
In principle MPC also represents a dosing threshold above which mutants should rarely arise; use of MPC would add consideration of the development of resistance to the traditional goal of clearing infection. With a clinical isolate of Mycobacterium tuberculosis
, we found that MPCs for two new C-8-methoxy fluoroquinolones are below the maximum attainable drug concentration in serum (6
). Thus the possibility exists that fluoroquinolones might be administered such that serum drug concentrations in patients exceed the MPC. Whether this is true for other quinolone-pathogen combinations and for large numbers of clinical isolates is unknown.
Examination of large numbers of clinical isolates generally involves measurement of antibiotic potency in terms of the MIC. With the agar dilution method, approximately 105
CFU is applied to each of a series of agar plates containing various antibiotic concentrations (18
). The concentration that allows no colony formation is taken as the MIC. Measurement of MPC is carried out using the same strategy except that more cells, on the order of 1010
, are applied to agar plates. Consequently, it should be possible to perform MPC measurements for a large number of clinical isolates. Those measurements could then be compared with published values of pharmacokinetic parameters to determine whether and for how long the serum drug concentration would be above the MPC. The relationship between pharmacokinetics and MPC could then be used to identify compounds for further examination of the inability to restrict selection of resistance.
In the present work, we tested five fluoroquinolones (gatifloxacin, grepafloxacin, levofloxacin, moxifloxacin, and trovafloxacin) with clinical isolates of S. pneumoniae. A hierachy of potency was clear. For the most active compounds, moxifloxacin and gatifloxacin, a provisional MPC (MPCpr), which overestimates MPC by about twofold, was below the maximum serum drug concentrations attained with the doses recommended for streptococcal pneumonia. Thus these fluoroquinolones may be useful in clinical trials to determine the utility of MPC in reducing the development of resistance. To facilitate further testing of MPC, we describe an empirical relationship that will allow MPC to be calculated from standard MIC measurements made by the agar dilution method.