Leon D. Sabath

Effect of Inoculum and of Beta-Lactamase on the Anti-Staphylococcal Activity of Thirteen Penicillins and Cephalosporins

Because there are few persuasive data for selecting one semisynthetic penicillin or cephalosporin over another for treatment of serious staphylococcal infections, 118 recent clinical isolates of Staphylococcus aureus were studied to determine to what extent the presence of β-lactamase affected the relative anti-staphylococcal activity of six penicillins and seven cephalosporins. In addition, the effect of inoculum was studied for its possible effect on the anti-staphylococcal activity of the 13 β-lactam antibiotics. By all criteria, methicillin and nafcillin were clearly more resistant to both the inoculum effect and the production of staphylococcal β-lactamase, whereas benzylpenicillin and cephaloridine (especially benzyl-penicillin) were the most susceptible to these effects. Cephazolin was clearly more susceptible to staphylococcal β-lactamase and heavy inocula than the other cephalosporins (with the exception of cephaloridine), whereas cephalothin was the most resistant cephalosporin to these factors. The minimal inhibitory concentration for benzylpenicillin for tests with undiluted inoculum, compared to results with inoculum diluted 10−4, differed by a factor up to 16,384, whereas with methicillin and nafcillin the differences were rarely more than twofold. Ratios for the other 10 antibiotics fell between these extremes. These results suggest that methicillin or nafcillin is most stable to staphylococcal β-lactamase, and that benzylpenicillin and cephaloridine are the most susceptible.

Susceptibility of Staphylococcus aureus and Staphylococcus epidermidis to 65 Antibiotics

The susceptibilities of 36 recent isolates of Staphylococcus aureus and 35 recent isolates of Staphylococcus epidermidis were determined against each of 65 antimicrobial agents and against two of them in combination. Rifampin was the most active of all the agents tested against both S. aureus and S. epidermidis. Among the penicillins, cloxacillin, dicloxacillin, and nafcillin were most active, although benzylpenicillin and phenoxymethyl penicillin were more active against susceptible strains. Cephaloridine was the most active of the cephalosporins, and sisomicin was the most active aminoglycoside. Minocycline was more active than the other tetracycline analogues tested. Among the macrolide-lincomycin compounds in clinical use, clindamycin was more active, and lincomycin was less active than erythromycin. The synergy of trimethoprim-sulfamethoxazole was more striking against S. aureus than against S. epidermidis. The median minimal inhibitory concentrations of the penicillins, cephalosporins, and aminoglycosides were lower against S. aureus, whereas the minimal inhibitory concentrations of the tetracyclines were lower against S. epidermidis.

Susceptibility of Nocardia asteroides to 45 Antimicrobial Agents In Vitro

A method for preparing uniformly dispersed cultures of Nocardia asteroides for use in tests for susceptibility to antimicrobial agents is described. The minimal inhibiting concentration (MIC) of 45 agents for cultures thus prepared was determined with the use of a replica-inoculating apparatus. Minocycline at a concentration of 3.1 μg or less/ml inhibited 90% of the strains tested, and all were inhibited by 6.3 μg/ml. An erythromycin concentration of 0.8 μg or less/ml inhibited 40% of the strains, but the MIC for most of the others was > 100 μg/ml. The other agents were generally less active. Chemically related analogues varied in activity to different degrees. Also, the MIC of each antibiotic against different strains generally varied over a wide range. Sulfonamides and trimethoprim were not active against most strains in the method used. The size of the inoculum markedly affected the MIC of sulfonamides and had variable effects on other agents. Marked synergy of erythromycin with ampicillin was demonstrated for nearly all strains tested.

Susceptibility of Beta-Hemolytic Streptococci to 65 Antibacterial Agents

Tests for susceptibility of 29 group A, 4 group C, and 2 group G strains of beta-hemolytic streptococci to 63 antibiotics and to trimethoprim and sulfamethoxazole, singly and combined in a ratio of 1:16, were carried out in vitro. All strains tested were moderately or highly susceptible to all the antibiotics used except those belonging to the aminoglycoside and polymyxin groups. A few were also resistant to the tetracyclines and to sulfamethoxazole alone. Comparisons with results obtained in previous years indicate that, except for the tetracyclines and sulfonamides, there has been no change in the susceptibility of beta-hemolytic streptococci to the most important and useful antibiotics, particularly penicillin.

Susceptibility of Pneumococci and Haemophilus influenzae to Antibacterial Agents

Strains of Diplococcus pneumoniae and Haemophilus influenzae were tested for susceptibility to numerous antibiotics by a twofold agar dilution method using an inocula replicator. Undiluted, fully grown broth cultures were used as inocula for both species, and cultures of pneumococci diluted 1:1,000 were also tested. The antibiotics included most of those in common use in the United States as well as some chemical modifications recently approved and others that are under investigation. The most striking aspect of the results was the marked susceptibility of the pneumococci to all the antibiotics tested except the polymyxins and most of the aminoglycoside antibiotics, although some new aminoglycosides were active in quite low concentrations. Some of the strains of pneumococci were of decreased susceptibility to penicillin G (minimal inhibitory concentrations, 0.2 to 0.4 μg/ml), but none were tetracycline resistant, although such strains had been reported previously from this laboratory. The strains of H. influenzae, which were all serologically nontypable, exhibited different patterns of susceptibility to the groups of antibiotics and to the individual chemically related ones. None of these strains (isolated early in 1972) were ampicillin resistant. The most active agents against H. influenzae were: carbenicillin and ampicillin, analogues related to each of them, rifampin, chloramphenicol, and the polymyxins. However, the tetracycline analogues other than tetracycline, some aminoglycosides, notably tobramycin, kanamycin, gentamicin, and verdamicin, erythromycin, and some new lincomycin analogues were also active in low concentrations. Trimethoprim alone was highly active, and in combination with sulfamethoxazole it was even more active and synergistic against strains of both D. pneumoniae and H. influenzae.

Suppression of Intrinsic Resistance to Methicillin and Other Penicillins in Staphylococcus aureus

The pH of the medium in which staphylococcal susceptibility to penicillins was determined was found to make a profound difference (128- to 8,000-fold) in the expression of “intrinsic” resistance, whereas β-lactamase-mediated resistance was only slightly affected by pH; methicillin-resistant staphylococci that are β-lactamase-negative are models of pure intrinsic resistance, and the common β-lactamase-producing organisms (methicillin-susceptible) are examples of pure β-lactamase-mediated resistance. Methicillin-resistant staphylococci were unable to express their resistance at pH 5.2. However, growth of methicillin-resistant organisms in acid (pH 5.2) medium, followed by susceptibility testing at pH 7.4, showed no elimination of the genotype for intrinsic resistance, indicating that the pH effect was due to suppression, rather than to elimination of the gene determining the intrinsic resistance. These pH changes had little effect on the susceptibility of staphylococci that possessed neither intrinsic resistance nor β-lactamase-mediated resistance. Thus, the suppression of “intrinsic” resistance was highly specific, and probably not the result of a change in ionization of the antibiotic, which would have been expected to affect all cells essentially equally. It is unlikely that foci of inflammation in man become sufficiently acid to suppress methicillin resistance of the staphylococci causing infection and inflammation.

Serum Glutamic Oxalacetic Transaminase: False Elevations during Administration of Erythromycin

Abstract Transient and sometimes marked elevations of serum glutamic oxalacetic transaminase were found in each of seven young adult male volunteers by a standard colorimetric assay after oral administration of one or two courses of 4 gm of erythromycin estolate. These elevations were shown to be due not to the enzyme but to an unidentified, trypsin-stable substance related to the administration of the antibiotic.

Drug resistance of bacteria.

DRUG resistance of bacteria is a major medical problem because it severely limits the usefulness of virtually all known antimicrobial agents and often necessitates the administration of highly toxi...

Increasing the usefulness of antibiotics: Treatment of infections caused by gram-negative bacilli.

Resistance of gram‐negative bacteria to many antibiotics often necessitates the use of relatively toxic antibiotics to treat infections caused by organisms that are resistant to less toxic ones. However, some relatively nontoxic antibiotics that appear to be ineffective against many gram‐negative bacilli by conventional tests can, in some instances, be made much more active by appropriate adjunctive measures. The measures studied were: (1) the use of analogues to inhibit destruction of antibiotic by bacterial enzymes (more specifically penicillinases) and (2) adfustment of the pH of the medium to maximize antibiotic activity. Both were applied to patients with infections caused by gram‐negative bacilli. Ampicillin plus a ±‐Iactamase inhibitor, cloxacillin, were used to treat a patient with endocarditis due to Enterobacter cloacae. Gentamicin was found to be 100 or more times as active in vitro at pH 8.5 as at pH 5.0 against most strains of gram‐negative bacilli that were studied. The concomitant use of bicarbonate or acetazolamide to allcalinize the urine permitted the use of this antibiotic in a much reduced dose.

Decrease in Ribosomal Density of Proteus mirabilis Exposed to Subinhibitory Concentrations of Ampicillin or Cephalothin

By conventional light microscopy, it has been shown that gram-negative bacilli exposed to subinhibitory concentrations of benzyl-penicillin become very much elongated and appear as filaments and whorls (1-5). Although it had been suggested that cephalosporins have the same modes of action as do penicillins (6), it has been shown (7, 8) that at subinhibitory concentrations penicillins cause a remarkable elongation (up to 93 μm), whereas cephalosporins cause globules and a minimal cell elongation of Proteus mirabilis (up to 14 μm). The present study was undertaken to determine whether transmission electron microscopy would reveal other morphological differences in the effects produced by subinhibitory concentrations of a penicillin (ampicillin) and a cephalosporin (cephalothin) on P. mirabilis 6. Materials and Methods. Exposure to antibiotics. P. mirabilis 6, originally isolated from the urine of a patient at Bronx-Lebanon Hospital, was grown in trypticase soy broth (TSB) (BBL) for 18 hr at 37°. The culture was diluted 1:100 in TSB and 0.1 ml was spread on a filter membrane PHWP09025 (Millipore Corp., Bedford, MA) and placed on the surface of trypticase soy agar (TSA) (BBL) in a petri dish. The plates with the inoculated membranes were incubated at 37° for 90 min, after which membranes were removed and transferred onto other TSA plates containing cephalothin (Lilly) or ampicillin (Bristol) at various concentrations and incubated for another 180 min. The antibiotic concentrations were equal to the minimum inhibitory concentration (MIC) as determined by an agar dilution technique (9) or to 2-fold dilutions of each MIC down to 1:16 MIC. The MICs were 1.56 μg/ml for ampicillin and 3.1 μg/ml for cephalothin.