Diffusion of trimethoprim and sulfamethoxazole from susceptibility disks into agar medium

The standard practice of using a single susceptibility disk for the antimicrobial combination Septra (trimethoprim/sulfamethoxazole) has been further justified by a direct measurement of the diffusion rates of each compound through agar medium. [(14)C]trimethoprim and [(35)S]sulfamethoxazole, singly and in combination, were applied to blank susceptibility disks which were incubated on 4-inch (10.16-cm) agar plates (Mueller-Hinton medium) at 37 C. The migration of each compound from the disk and diffusion through agar were measured with time by determining the radioactivity in concentric zones extending from the origin. The two compounds diffuse with similar rates, maintaining approximately a 1:20 concentration ratio which is approximately the ratio of trimethoprim to sulfamethoxazole observed in plasma during treatment. The diffusion rate is independent of the presence of the other compound; greater than 95% of the radioactivity is transferred from the disk to the agar in 24 h.

Letter: Co-trimoxazole resistance

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New system for cultivation of Neisseria gonorrhoeae

A modified Thayer-Martin medium, containing 2.0% agar, 0.25% dextrose, and 5.0 mug of trimethoprim per ml was compared to Thayer-Martin medium for growth of stock cultures of Neisseria gonorrhoeae and for isolation of N. gonorrhoeae from clinical specimens. Stock cultures and male urethral cultures grew equally well on both media, but the modified Thayer-Martin medium detected 10% more female patients positive for gonococci. A description is given of the use of a citric acid-sodium bicarbonate tablet for generating carbon dioxide in a sealed plastic bag containing a culture plate. Equivalent results were obtained on duplicate plates of 336 cultures and specimens when this procedure was compared with the standard candle jar extinction method of incubation. The tablet and plastic bag system has the advantages of economy, ease of use, and safety.

Selective media in the isolation of tubercle bacilli from tissues

Direct culture on Lowenstein-Jensen slopes and on three media made selective for tubercle bacilli by the addition of four antibacterial agents was compared with guinea-pig inoculation on 490 tissue specimens. Tubercle bacilli were obtained from 15 specimens by culture and 14 by guinea-pig inoculation; only one specimen was positive by guinea-pig and not by culture. The most efficient culture medium was a selective 7H11 slope. Routine guinea-pig inoculation has been replaced by a wider range of culture procedures.

Obligate thymidine auxotrophs of Pseudomonas acidovorans

The aminopterin technique was adapted for the isolation of thymidine auxotrophs of Pseudomonas acidovorans. All the mutants isolated responded to thymidine but not to thymine.

Thymineless death and ultraviolet sensitivity in Micrococcus radiodurans

Thymine-requiring mutants of Micrococcus radiodurans have been isolated by selection on solid medium containing trimethoprim. Strains requiring either high concentrations of thymine (50 mug/ml) or low concentrations (2 mug/ml) for normal growth were obtained. The Thy(-) mutant requiring low thymine concentrations has been characterized. It was shown to retain the high ultraviolet light (UV) resistance typical of wild-type M. radiodurans, but it was not resistant to thymineless death. Preliminary exposure of the cells to thymineless conditions resulted in enhanced UV sensitivity, and this interaction occurred under conditions where "unbalanced growth" was inhibited by the addition of chloramphenicol. Upon addition of thymine to deprived cells, UV resistance was gradually restored, and this recovery took place in the absence of protein synthesis. A model is proposed to account for the similarity of thymineless death in bacteria whose deoxyribonucleic acid repair efficiencies differ widely.

The control of ribonucleic acid synthesis in bacteria. Polymerization rates for ribonucleic acids in amino acid-starved relaxed and stringent auxotrophs of Escherichia coli

Polymerization rates of newly formed chains of various RNA fractions were measured in Escherichia coli CP78 (RC(str)) and CP79 (RC(rel)) multiple amino acid auxotrophs, deprived of four amino acids essential for growth. Immediately after the onset of severe amino acid deprivation, in RC(str) strains the rate of labelling of RNA by exogenous nucleotide bases was greatly diminished. At first, the initiation of new RNA chains declined faster than the rate of polymerization in RC(str) organisms, but as starvation proceeded the rate of polymerization was eventually lowered to about 10% of that found during normal growth. In strain CP79 (RC(rel)), on the other hand, chain-polymerization rates were unaffected by amino acid withdrawal. Artificial depletion of the intracellular purine nucleotide pools in RC(str) or RC(rel) strains by trimethoprim, before the onset of amino acid deprivation, showed that in the RC(str), but not the RC(rel) strain, amino acid withdrawal gave rise to an inability of the cells to utilize exogenously supplied purine or pyrimidine bases for RNA synthesis. During a prolonged starvation, the observed 100-fold decrease in the total rate of incorporation of exogenous nucleotide bases into the RNA of RC(str) organisms was ascribed to a combination of a tenfold decrease in the overall rate of RNA chain polymerization, at least a fivefold decrease in the ability of the cells to utilize exogenous bases and a preferential inhibition of initiation of stable RNA chains. None of these changes occurred in the corresponding RC(rel) strain.

Neisseria gonorrhoeae: colonial morphology of rectal isolates

Four principal colony types of gonococci have been previously described, and it has been shown that primary isolates from the urethra and cervix are primarily of colony types 1 and 2. In the present work, gonococcal isolates from the rectum were also shown to be predominantly colony types 1 and 2. Visualization and typing of gonococcal colonies in primary rectal isolates were facilitated by the use of medium containing vancomycin, colistin, nystatin, and, in some cases, trimethoprim lactate. Control experiments showed that these agents sometimes caused minor alterations in colonial morphology; but with knowledge of these alterations satisfactory colonial typing could be made.