The fluoroquinolones, particularly ciprofloxacin, have been suggested to treat methicillin-resistant Staphylococcus aureus (MRSA) infections and colonization and methicillin-susceptible S. aureus (MSSA) infections. The development of ciprofloxacin resistance in MRSA and MSSA was prospectively evaluated. After 3 months of ciprofloxacin use, high-level resistance (MIC90, 64 micrograms/ml) developed in MRSA and increased at an alarming rate, from none to 79% over a 1-year period. High-level ciprofloxacin resistance also developed in MSSA, increasing to 13.6% over the same period. Antibiograms, phage typing, and plasmid profile analysis suggest that more than one clone of MRSA developed resistance and that ciprofloxacin resistance is not associated with the acquisition of a new plasmid. Most patients had nosocomial acquisition and about one-half had a history of previous ciprofloxacin use. Ciprofloxacin resistance can develop rapidly in S. aureus; thus, ciprofloxacin appears to have limited usefulness in treating staphylococcal infections and colonization, especially those due to MRSA.

Variation in the chromosomal genomes of newport) in isolates of clones belonging to several evolutionary lineages, some of which are distantly related, suggests that the horizontal transfer and recombination of chromosomal genes mediating expression of cell-surface antigens has been a significant process in the evolution of the salmonellae. Two divergent clone clusters of S. derby differ in the relative frequency with which they cause disease in birds versus mammals, and two major lineages of S. newport differ in the frequency with which their clones are associated with disease in humans versus animals.

We compared four phenotypic and six genotypic methods for distinguishing Campylobacter jejuni strains from animals and humans involved in four epidemics. Based on a comparison with epidemiologic data, the methods that correctly identified all strains in three milkborne outbreaks and one waterborne outbreak were heat-stable and heat-labile serotyping; multilocus enzyme electrophoresis (MEE); DNA restriction endonuclease analysis with BgIII, XhoI, PvuII, or PstI; and Southern blot and hybridization of PvuII-and PstI-digested DNA with Escherichia coli 16S and 23S rRNA (ribotyping). Biotyping, phage typing, plasmid analysis, and probing of BglII and XhoI DNA digests with C. jejuni 16S rRNA genes failed to correctly separate one or more strains. MEE, restriction endonuclease analysis, and ribotyping were the most sensitive methods and identified nine types among the 22 strains. These methods were also capable of further distinguishing strains within the same serotype. Data from MEE were also analyzed to calculate genetic relatedness among strains. Serotyping was the most discriminating phenotypic method, with eight and seven types distinguished by the heat-stable and heat-labile methods, respectively. MEE and ribotyping had several advantages over the other methods because they measure relatively stable and significant chromosomal differences and are applicable to other species and genera. These methods, however, are complex and not easily quantified; they are currently limited to specialized laboratories. When antisera are available, serotyping appears to be an effective and more practical approach to the identification of epidemic-related strains.

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Analysis of DNA restriction fragment length polymorphisms of rRNA genes (ribotyping) was employed to assist in the epidemiologic investigation of the emergence and spread of ciprofloxacin-resistant Staphylococcus aureus at the Atlanta VA Medical Center because many isolates of interest were nontypeable by phages and harbored few plasmids useful as strain markers. Chromosomal DNAs of selected S. aureus isolates were digested initially with 20 different restriction enzymes. EcoRI appeared to give the best discrimination of hybridization banding patterns (ribotypes) and was used with all study isolates. Overall, 15 different ribotypes were seen among the 50 S. aureus isolates studied (7 ribotypes among 13 methiciUlin-susceptible S. aureus [MSSA] isolates and 9 ribotypes among 37 methicillin-resistant S. aureus [MRSA] isolates). Seven of eight ciprofloxacin-resistant MSSA (CR-MSSA) patient isolates had identical antibiograms, were nontypeable by phages, and had a single 22-MDa plasmid. Six of these seven CR-MSSA isolates had an identical ribotype pattern. Ribotyping distinguished this CR-MSSA strain or clone from MRSA and other MSSA isolates, including nontypeable isolates that contained a 22-MDa plasmid. Five ciprofloxacin-susceptible MSSA isolates studied had five ribotypes; one pattern was identical to the CR-MSSA clone. Twenty-three CR-MRSA isolates recovered from the Atlanta VA Medical Center had four different ribotypes. Ribotyping proved to be a useful molecular