Effects of subminimal inhibitory concentrations of chloramphenicol, erythromycin and penicillin on group A streptococci

Postantibiotic effect of penicillin on Streptococcus anginosus

We studied the postantibiotic effect of penicillin G on bacterial growth of two strains of Streptococcus anginosus by optical density readings of the cultures and by counting the numbers of viable cells. Duration of the effect of the drug in concentrations equivalent to the MICs after exposure for 2 h was 3.4 and 3.5 h. The production of streptococcal substances was examined during the postantibiotic phase. The antibiotic caused an increase in deoxyribonuclease and a decrease in both free and cell-bound hemolysin activities of one strain. The other strain displayed an increase in hyaluronidase and both free and bound hemolysin production.

Correlation between growth in antibiotic-medium and hemolytic activity of group C and G streptococci

The effect of a subminimal inhibitory concentration of penicillin on the production of bound and free hemolysins by streptococci was examined using sheep red blood cells. A marked decrease of a group C cell-free and bound activities was observed with penicillin at a concentration of 1/3 of the MIC whereas an increase was observed with those of a group G strain. Potassium ferricyanide and anti-streptolysin O (group A streptococcus) were strongly inhibitory for the free activities of both strains. The cell-bound activities were stimulated by addition of RNA during bacterial growth in control cultures and also in drug-containing media.

Penicillin and clindamycin alter some group A streptococcal products

The effects of subMICs of penicillin and clindamycin on group A streptococcal products were analyzed. Penicillin caused an increase in the expression of the group carbohydrate for the three strains tested; in contrast, clindamycin inhibited the expression in two strains. The M and T proteins were not affected by the drugs. Clindamycin caused an increase in the hyaluronidase activity and both antimicrobial agents inhibited the activity of streptolysin at 1/2 MIC.

Effect of ciprofloxacin on phagocytosis

Certain aspects of the relationship between host defence mechanisms and the new quinoline derivative ciprofloxacin in comparison to norfloxacin and ofloxacin were studied. Ciprofloxacin did not affect chemotaxis of human polymorphonuclear leucocytes in agarose. In leucocytes exposed to ciprofloxacin, norfloxacin and ofloxacin neither the chemiluminescent response to opsonized zymosan and formyl-methionyl-leucyl-phenylalanine nor the phagocytic or bactericidal activity was affected. However, killing of Staphylococcus aureus by human polymorphonuclear leucocytes exposed to subinhibitory concentrations of ciprofloxacin was enhanced. The results show that the quinolines tested do not directly influence phagocytic cells, but a subinhibitory concentration can make bacteria more susceptible to phagocytosis and killing.

Erythromycin: a microbial and clinical perspective after 30 years of clinical use (1)

Erythromycin is a macrolide that acts by inhibiting the translocation reaction during protein synthesis. Erythromycin is inactive against the Enterobacteriaceae and Pseudomonas aeruginosa except under alkaline conditions. Erythromycin is active against most gram-positive bacteria; some gram-negative bacteria, including Neisseria, Bordetella, Brucella, Campylobacter, and Legionella; and Treponema, Chlamydia, and Mycoplasma. The emergence of resistance to erythromycin is closely associated with its use and is often plasmid mediated. After its oral or parenteral administration, erythromycin diffuses readily into intracellular fluids and is actively concentrated intracellularly by polymorphonuclear leukocytes and alveolar macrophages.