In vitro activity of four fluoroquinolones on selected bacteria

In 1990-1991, in a national surveillance study, and in 1991-1992, in a followup study, both by Thornsberry et al. (1993), ciprofloxacin data from various geographical and demographical institutions were collected. Several species of bacteria have shown resistance to ciprofloxacin and norfloxacin, but the degree of resistance to these drugs has not been reported for the Stockton area. To determine the extent of this resistance, Dameron Hospital antibiograms generated from 1990 to 1994 were reviewed and compared. Results of the comparison show that susceptibility among the Gram-negative isolates, with the exception of Providencia stuartii, Acinetobacter lwoffi, and to a lesser extent Aeromonas hydrophila, has changed very little. Consistent with the national surveys, resistance of Pseudomonas aemginosa has not changed appreciably during the five-year period.Among the Gram-positive isolates that were tested against both ciprofloxacin for a five-year period (1990-1994) and norfloxacin for a three-year period (1992"' 1994), increased resistance was seen among strains of Staphylococcus aureus, S. epidermidis, S. haemolyticus, and Enterococcus jaecalis, but not among strains of Staphylococcus saprophyticus, Streptococcus pyogenes, and S. agalactiae. To determine whether resistance to one fluoroquinolone occurs also to other fluoroquinolones, several isolates of Gram-positive cocci and P. aeruginosa from the Gram-negative bacilli that showed resistance to either ciprofloxacin, norfloxacin, or both were selected from Dameron Hospital isolates and tested by the disk diffusion technique against ciprofloxacin, norfloxacin, ofloxacin, and lomefloxacin. The results indicate that differences do exist among these selected strains. Comparison of the invitro effectiveness of the various quinolones confirms that methicillin-resistant staphylococci (S. aureus, S. epidermidis, and S. haemolyticus) exhibit a higher degree of resistance to the four fluoroquinolones compared with the methicillin-susceptible strains of the same species. Resistance of the enterococci (Enterococcus jaecalis and E. jaecium) is also high. Generally, when the four fluoroquinolones were compared with each other, ofloxacin seemed to have better in vitro activity.Resistance to the quinolones consists of two proposed mechanisms: ( 1) mutation of one or both of the structural genes of the A and B subunits of DNA gyrase and (2) decreased drug accumulation due either to lower uptake by the cell or enhanced effiux out of the cell. These mechanisms of resistance are reviewed.