An epidemic strain of methicillin-resistant Staphylococcus aureus (MRSA) isolated from patients at the University of Virginia Hospital in Charlottesville (MRSA-Va) was characterized, and virulence properties were compared with those of three clinically significant strains of methicillin-sensitive S. aureus. Unlike most known strains of MRSA, MRSA-Va was sensitive to tetracycline and streptomycin and exhibited high-level homogeneous methicillin resistance (minimal inhibitory concentration, greater than 128 microgram/ml). The expression of resistance was not influenced by incubation temperature. MRSA-Va contained significantly more catalase (P less than 0.05) than methicillin-sensitive strains but about the same amount of protein A. Phagocytosis and killing of MRSA-Va by normal polymorphonuclear leukocytes were not significantly different from that of the Wood-46 strain of S. aureus. In mouse virulence studies using both intraperitoneal and intravenous modes of infection, 50% lethal doses for MRSA-Va were comparable with those of the three methicillin-sensitive strains. This epidemic strain of MRSA appears to be fully virulent.

Primary cultures of human endometrial and ectocervical epithelial cells were examined as a new model system to study genital infection by Chlamydia trachomatis. Initial studies demonstrated that these cells were indeed susceptible to chlamydial infection. Inocula, adjusted to produce inclusions in 50 to 80% of equivalent numbers of standard McCoy cells, resulted in infection rates of approximately 15 to 30% for the columnar cells of the endometrium and 5 to 10% for the squamous cells of the ectocervix. Exposure of cultures to DEAE-dextran and centrifugationassisted inoculation, manipulations reported to enhance infection of HeLa and McCoy cells, did not alter the number of inclusion-positive genital cells. Addition of cycloheximide to the postinoculation culture medium slightly increased numbers of inclusion-bearing cells while growth of genital cells in hormone-supplemented medium resulted in a variable effect on inclusion development and a significant reduction in the association of radiolabelled organisms with these cells. The basis for the different levels of infection in McCoy versus genital cell cultures was revealed by immunofluorescence analysis of chlamydia1 association with host cells immediately after inoculation. Chlamydiae failed to adhere to many cells in the genital cell cultures while adherence to McCoy cells was uniform. In addition, the association of radiolabelled C. trachomatis was significantly lower with genital cells than with McCoy cells. Finally, culture conditions were defined which markedly inhibited inclusion development without an immediate loss of chlamydial growth potential. This investigation indicates that primary genital cell cultures are susceptible to chlamydial infection and will be valuable for studies on the nature of C. trachomatis interactions with natural human target cells.

Rifampin is a potentially useful anti-staphylococcal agent, but resistance develops frequently when the drug is used alone. The efficacy of rifampin, trimethoprim, and a penicillin alone or in combination was examined in mice with acute or subacute infections. Mice were infected intraperitoneally with penicillin-susceptible Staphylococcus aureus. Survival after penicillin therapy was only 9.1% in contrast to survival after rifampin therapy which was 68% (P < 0.001).No rifampin-resistant S. aureus were isolated from peritoneal fluid or heart blood samples from dead animals in these short-term experiments. Rifampin was ineffective (survival, 4.8%) for infections instituted with rifampin-resistant strains. Long-term experiments were conducted after intravenous injection of 4 x 108 S. aureus. Forty percent of the animals survived after methicillin therapy; 77% survived after rifampin therapy (P < 0.001). However, 40% of those animals that died after rifampin therapy died with rifampin-resistant organisms. No animal dying in groups treated with a combination of rifampin and trimethoprim (85% survival) or rifampin and methicillin (79% survival) died with rifampin-resistant organisms. Thus, rifampin combined with a penicillin or trimethoprim was effective in preventing the development of rifampin-resistant strains.

Twenty clinical isolates of Staphylococcus aureus were examined to determine the frequency of rifampin-resistant variants. All isolates were highly susceptible to rifampin, with mean minimum inhibitory concentrations of 0.11 ± 0.1 ,ug/ml and mean minimum bactericidal concentrations of 0.22 ± 0.2 ,ug/ml. The frequency of isolation of resistant variants was similar at all rifampin concentrations tested. Rifampin-resistant variants maintained their resistance upon daily subculture in rifampin-free broth. Rifampin-susceptible S. aureus exhibited a growth and survival advantage over the rifampin-resistant mutants both in pure cultures and in mixtures with rifampin-resistant antecedents. A comparison of the virulence for mice of five susceptible isolates and their 100 ,ug/ml-rifampin-resistant variants showed that two of the resistant variants were less virulent than the susceptible strains via intraperitoneal challenge, whereas three of the resistant variants were less virulent by intravenous challenge (P < 0.05).Rifampin is a potent antistaphylococcal agent, highly efficacious for the treatment of experimental staphylococcal infections (2,9,11,14). Consequently, rifampin is being used with increasing frequency for the therapy of selected staphylococcal infections in humans (1,3,4,13,18). However, rifampin-resistant organisms readily develop, and this may compromise the continuing therapeutic applications of the drug (11,14,15,17).The present study examines the frequency of resistance in 20 clinical isolates of Staphylococcus aureus to several concentrations of rifampin. The stability and growth of rifampin-resistant variants in vitro were measured, and the virulence of these variants was determined in mice.MATERIALS AND METHODS

The efficacy of six antistaphylococcal agents for the treatment of experimental methicillin-resistant Staphylococcus aureusinfections was studied. Mice were injected intravenously with ≈ 5 × 10⁸ methicillin-resistant S. aureus and treated by intraperitoneal administration of antimicrobics. Vancomycin, trimethoprim-sulfamethoxazole and rifampin therapies resulted in increased survival when compared to untreated controls.