Antimicrobial combination therapy may be used to extend spectrum coverage, prevent the emergence of resistant mutants and gain synergy between antimicrobials [11
]. Combination therapy is often recommended for empirical treatment of bacterial infections in intensive care units, where monotherapy is not likely to cover all potential pathogens, and the emergence of resistance is a potential threat [20
]. In the last years, development of new antimicrobials targeted to treatment of MRSA infections, such as linezolid, has led to assess antibacterial activity of such drugs in combination with several antibacterials, particularly β-lactams [21
]. Also combination of vancomycin with aminoglycoside or β-lactams has been widely investigated, generally reporting synergy with β-lactams while no interactions of even antagonism were observed with aminoglycosides [25
]. Activity of combinations containing teicoplanin against S. aureus
has been less investigated [27
]. The present study has compared antimicrobial combinations containing teicoplanin or vancomycin for their antibacterial activity and ability to select for mutants able to grow in their presence in nosocomial MRSA. To our knowledge this is the first study evaluating the efficacy of triple combinations of a glycopeptide, a β-lactam and a fluoroquinolone against MRSA strains. Triple combinations with vancomycin have been recently indicated by American Thoracic Society and Infectious Diseases Society of America for initial empiric therapy for hospital acquired pneumonia, ventilator associated pneumonia and healthcare associated pneumonia in patients with potential MRSA etiology [13
]. These guidelines suggest the use of vancomycin or, as alternative, of linezolid, combined with one among antipseudomonal cephalosporins or carbapenems or β-lactam/β-lactamase inhibitor (i.e. piperacillin-tazobactam) and an antipseudomonal fluoroquinolone or an aminoglycoside. Teicoplanin is not included, since it has not been approved in the United States, however, in all countries where it has been approved, teicoplanin represents an alternative to vancomycin [13
]. In this study, data from the checkerboard assay indicates that teicoplanin with cefotaxime showed the highest rate of synergy among the tested combinations, followed by those with levofloxacin and ceftazidime or piperacillin/tazobactam. Ability of β-lactams in triple combinations to yield synergy was quite different for the various molecules, ranging from 20 to 80%.
Killing curves confirmed superior activity of teicoplanin plus cefotaxime, while smoothed differences among β-lactams. Since in in vitro
experiments glycopeptides are slowly bactericidal, it may be hypothesized that their antistaphylococcal bactericidal activity could be increased by use of combinations [29
]. It is also evident that this ability was not a class effect since marked differences were found among β-lactams tested and between the two glycopeptides. Therefore it seems rather difficult to compare our data with those of other studies in which different β-lactams have been used.
Methicillin resistance is known to favor development of multi-drug resistance, including macrolides and fluoroquinolones. In fact, for the strains evaluated in this study, determination of frequency of mutation for each antibiotic in single was not always possible, since they were already resistant to the molecules tested and therefore able to grow at antibiotic concentrations equal to the resistant breakpoint. Data obtained indicate that glycopeptides in double or triple combinations were effective in preventing the emergence of mutants able to grow in presence of the antibiotics. No mutants were isolated in vitro
after incubation with glycopeptides alone, thus correlating the low frequency of development of glycopeptides resistance observed in vivo
. Different results have been reported for teicoplanin and vancomycin after serial exposure to these antibiotics by other authors, but the differences between the methods employed are likely the cause of this discrepancy [30
]. However, further studies on development of resistance possibly induced by the tested antibiotics in single and in combinations after serial passages are now in progress.