The susceptibility of 100 group B streptococci to 16 ,-lactam antibiotics was tested by agar dilution. Penicillin G and N-formimidoyl thienamycin were the most active agents tested, both having a 90% minimal inhibitory concentration (MIC90) of 0.06 ,ug/ml. Ceftriaxone, cefotaxime, cefamandole, and SCH 29482 were almost as active, all having an MIC90 of 0.12 ,ug/ml, and ampicillin, cephalothin, and mezlocillin all had an MIC%0 of 0.25 ,ug/ml. The MIC90 for piperacillin, cefoperazone, and ceftazidime was 0.5 jig/ml. Least (Schering Corp.). These agents were supplied as laboratory standard powders of known potency, and stock solutions were made as recommended by the manufacturers.MICs were determined by the agar incorporation method in plastic petri dishes (15 by 100 mm) containing 25 ml of Mueller-Hinton agar supplemented with 5% sheep blood and incorporating the above antimicrobial agents in concentrations of 0.008 to 256 ,ug/ml in doubling dilutions (13). After pouring, the plates were dried, wrapped in plastic, and refrigerated at 4°C before use within 7 days.For MIC testing, isolates were grown in 2 ml of Mueller-Hinton broth at 35°C for 3 to 5 h, and the turbidity was then adjusted to a turbidity equivalent to that of a 0.5 McFarland barium sulfate standard with a nephelometer (1R, API; Analytab Products). Suspensions were further diluted 1:20 to obtain a final inoculum of 104 organisms per 2 ,u. Plates were inoculated with a Steers replicator with 3-mm inoculating pins and inoculated overnight at 35°C in room air. The MIC was defined as the lowest concentration of antibiotic completely inhibiting the growth of each isolate. Quality control was ensured if results obtained with recommended controls (13)

No abstract

West , B. (University of Oklahoma Medical Center, Oklahoma City), Florene C. Kelly, and Doris A. Shields . Effect of sodium chloride on staphylococcus-phage relationships. J. Bacteriol. 86: 773–780. 1963.—Phage patterns of 21 phage-propagating strains of staphylococci on medium with high NaCl content appeared to be an expression of the staphylococcal cells, as well as of the salt tolerance of the phages. Serological group A phages, previously found to be NaCl-tolerant in the free state, were capable of lysing susceptible staphylococci on 3, 7.5, and 10% NaCl Trypticase Soy Agar. None of the other phages tested was active when the medium contained 7.5 and 10% NaCl. Increasing the NaCl content of the medium rarely resulted in nonspecific reactions; rather the effect was, generally, a narrowing of the phage spectrum of the cells, with persistence in the phage pattern of the phage, or phages, which were propagated on the cells being tested. Although NaCl tolerance of the phages was the chief limiting factor of phage activity in the presence of 7.5 and 10% NaCl, reactions on salt medium also depended on the degree of susceptibility of cells to phage on routine typing medium and to certain other unexplained factors. In some instances, under the influence of increased NaCl, significant lysis at 1000 RTD was replaced by thinning of growth (inhibition), with or without the presence of plaques. Conversely, certain phage-cell combinations, which gave inhibition at 1000 RTD on standard medium produced some degree of lysis when the NaCl concentration was increased. Studies of phage 81 and its propagating strain showed that replication of phage occurred in 10% NaCl medium, although adsorption diminished as salt concentration was increased, and the time required to reach maximal lytic activity was delayed.

Of the methods now available for the detection of coagulase positive staphylococei, none is designed to show both clumping factor and free coagulase activity in a single system. The observations reported here are by-products of a search for variant strains of staphylococci with particular reference to strain differences in the cell surface component known as clumping factor or bound coagulase. In this search, the soft agar technique, which has been applied to variation studies by a number of workers (McCarty et al., 1946; Pike, 1946; Pittman and Davis, 1950; Lankford et al., 1955; Finkelstein and Sulkin, 1958), has proved most rewarding. A conmparison of the reactions of staphylococei in various soft agar media with the results of conventional coagulase and agglutination tests led to the method described here for simultaneous detection

SUMMARYTwenty-two staphylococcal bacteriophages representing serological groups A, B, F and L were examined for activity after incubation of free phage in 10 yo (w/v) NaCl Trypticase soy broth for 24 hr. at 30". The mean decrease in plaque-forming units for ten phages of serological group A was 13 yo; in no instance was inactivation of any group A phage greater than 33 yo. A11 but one of nine group B phages were 88 % or more inactive after incubation in the 10 yo NaCl broth; the single exception was phage 83 with slightly more than 40% inactivation in both 0.5y0 or 10% NaCl broth. Serological group F phage 42D resembled phages of group A, whereas phage 77 of group F and group L phage 187 were intermediate between group A and B phages with respect to NaCl stability.