Effects of Low Penicillin Concentrations on Cell Morphology and on Peptidoglycan and Protein Synthesis in a Tolerant Streptococcus Strain

Abstract

Rates of protein and peptidoglycan synthesis were determined by pulse-labeling techniques before and after treatment of exponentially growing cultures of Streptococcus mutans FA-1 with a number of concentrations of penicillin G (0.05, 0.1, 0.3, and 0.4 pg/ml). These penicillin concentrations were all less than that required to saturate the specific penicillin-binding sites present on the surface of this organism (0.5 ug/ml), but were all greater than and, in fact, were multiples of the minimum inhibitory concentration (0.02 ug/ml). Low concentrations of penicillin G (2.5x the minimum inhibitory concentration) immediately halted the exponential increase in the rate of peptidoglycan synthesis normally expected as the result of cell multiplication, but allowed the rate of peptidoglycan synthesis occurring at the time of penicillin addition to be maintained for almost 1 h. An increased penicillin concentration (5x the minimum inhibitory concentration) allowed the rate of peptidoglycan synthesis occurring at the time of penicillin addition to be maintained for a shorter length of time (-0.67 h). Still greater penicillin concentrations caused an immediate inhibition of the peptidoglycan synthetic rate. The effect of penicillin on the rate of protein synthesis was similar, although less pronounced. Samples were taken for scanning electron microscopy immediately before and after 3 h of treatment with a low (2.5x the minimum inhibitory concentration) concentration of penicillin. The surface areas and volumes of the cells in these samples were calculated from the electron micrographs by using computer reconstruction techniques. From the frequency distributions of surface area, the plots of surface area to volume ratio as a function of surface area, and the pulse-labeling data mentioned previously, low, growth-inhibitory concentrations (2.5x the minimum inhibitory concentration) of penicillin are proposed (i) to inhibit the constriction of the division septum, (ii) to prevent the establishment or maturation of new envelope growth sites, and (iii) to have no immediate effects on the synthesis of cell wall peptidoglycan already in progress at the time of penicillin addition.Streptococus mutans FA-1 has recently (18) been described as an organism which is tolerant to the lethal and lytic effects that often follow the inhibition of cell wall peptidoglycan (PG) assembly (1,21,22), although it is highly susceptible to growth inhibition by several antibiotics that inhibit PG synthesis. Penicillin G (Pen G) is a structural analog of D-alanyl-D-alanine (26) and is for this reason thought to inhibit the transpeptidation step of PG assembly, although examples are known oforganisms in which transpeptidation is not susceptible to penicillin inhibition (3, 9).The standard methods used to measure the efficacy with which an antibiotic interferes with bacterial metabolisn have been quantitation of the antibiotic concentration necesary to inhibit growth (minimum inhibitory concentration [MIC]) and the concentration of that antibiotic necess...

Oxacillin-induced inhibition of protein and RNA synthesis in a tolerant Staphylococcus aureus isolate

“…Standard clinical testing of these organisms shows that the MBC is much greater than the MIC. The biochemical mechanism of this so-called tolerant (12) response of certain strains to normally bactericidal antibiotics is not known; however, one possible explanation suggests the presence, in tolerant strains, of a special form of regulatory "cross talk" such that the almost immediate consequence after inhibition of peptidoglycan synthesis by a beta-lactam antibiotic is the inhibition of protein synthesis (9,10). This inhibition of protein synthesis was postulated to antagonize whatever lytic signal was sent to the cell wall autolytic system of the tolerant bacterium by the beta-lactam antibiotic in the same way that inhibitors of protein synthesis are known to antagonize.…”