This study investigated the use of various beta-lactam discs in the detection of penicillin sensitive, intermediate resistant and resistant strains (minimal inhibitory concentrations of penicillin G of less than 0.06, 0.06 to 0.25, and greater than 0.25 mg/l respectively). Discs containing 0.15 and 0.6 microgram of penicillin G, and 5 microgram of methicillin were the most useful in separating the three groups. Cut-off zones between sensitive, intermediate resistant and resistant strains are 12 and 24 mm for 0.15 microgram penicillin discs; 24 and 32 mm for 0.6 microgram penicillin discs; and 20 and 30 mm for 5 microgram methicillin discs. Oxacillin discs (1 microgram) were useful in separating resistant from both sensitive and intermediate resistant strains at a cut-off zone of 8 mm. Use of a combination of discs provides the best index of susceptibility group, and at least two should be used

A group Hi R factor encoding resistance to chloramphenicol, streptomycin, sulfonamide, and tetracycline was transferred into Salmonella typhimurium LT-2. The virulence of LT-2 for mice, as assessed by intraperitoneal 50% lethal dose and the number of organisms in the spleen, was not affected by the R factor. On the other hand, the R factor conferred resistance in mouse infections to therapy with chloramphenicol and trimethoprim plus sulfamethoxazole.The epidemic of R factor-mediated typhoid fever in Mexico in 1972 and 1973 affected an estimated 10,000 to 15,000 persons with disease that was more severe and protracted with a higher rate of complications than usually seen with typhoid fever (11,13,15). These features led to speculation that the R factor may have increased the virulence of Salmonella typhi (11). At the same time, antibiotic-resistant typhoid fever in Vietnam was shown to be associated with an identical R factor (7), which was classified as belonging to group Hi (1). Chloramphenicol has traditionally been the drug of choice (10), but it was ineffective in treating patients in Mexico and Vietnam (4,6,7,13,18). Ampicillin and amoxicillin were used in these epidemics with good results (6,8,12,17), but there is disagreement over the efficacy of the combination of trimethoprim and sulfamethoxazole (TMP-SMZ) (6, 12). In the following studies, murine typhoid infection (2, 3, 9, 14) was employed to assess the effect of the R factor from a drug-resistant strain of S. typhi on the virulence of the infecting agent and the response of infections to antimicrobial therapies currently used in the treatment of typhoid fever.The laboratory strain Salmonella typhimurium LT-2 was used in all experiments to receive the R factor and to infect mice. S. typhi resistant to chloramphenicol, streptomycin, sulfonamide, and tetracycline was isolated from the blood of a patient with typhoid fever in Vietnam in 1975 (patient 124) (6). This strain and others from Vietnam had been shown by E. S. Anderson to harbor a group H1 R factor (1, 6). The R plasmid was transferred by conjugation to the Escherichia coli K-12 strain EB 50 proB metB trpA his Strr Nalr thy. Exponentially growing cultures of the donor and recipient strains were mixed in nutrient broth at a concentration of 2 x 10' cells per ml and incubated for 2 h. The mating mixture was plated on nutrient agar containing chloramphenicol (30 ,ug/ml) and nalidixic acid (80 ,tg/ml). The transconjugant colonies were restreaked twice on selective medium, and the plasmid was transferred to LT-2 by conjugation by using the above procedure, with the exception that mating was allowed to procede overnight. R factor-containing colonies of LT-2 were selected on minimal salts agar containing glucose (0.5%) and chloramphenicol (30 ug/ml).These colonies were reisolated on minimal selective agar twice before testing for multiple antibiotic resistance by disk susceptibility. The strains, both with and without R factor were susceptible to ampicillin (minimal inhibitory concentration, 2.5 tLg/...