Antimicrobial resistance: a class effect?

Antibiotic use has led to increased resistance to certain group markers: penicillin, erythromycin and ciprofloxacin for beta-lactams, macrolides and quinolones, respectively. The influence of resistance to markers in decreasing susceptibility to the drugs included (on the basis of chemical structure) in the corresponding antibiotic group can be defined as 'resistance class effect'. In the case of macrolides, this effect is dependent on the prevalent resistant phenotype among the isolates of the target bacteria: the class effect exists completely if the mechanism of resistance is constitutive MLS(B) (all macrolides are affected by resistance to erythromycin), and only partially if the mechanism is the efflux M phenotype (all but 16-membered macrolides are affected). In Spain, the first case is exemplified by Streptococcus pneumoniae and the second by Streptococcus pyogenes. For beta-lactams and quinolones, resistance to the group markers results in large decreases in the antimicrobial activity of the less potent members of the group, penicillin being a better driver of resistance for oral cephalosporins than for aminopenicillins, and ciprofloxacin being a better driver for older rather than for the newer quinolones, which have enhanced anti-pneumococcal activity. Empirical prescription guidelines based on the pharmacoepidemiology of resistance, recommending the use of potent drugs that are less influenced by resistance to the marker, may help to counter the spread of resistance in the community.