Altered Cell Walls of Staphylococcus aureus resistant to Methicillin

Most studies on pharmacodynamic variables in vitro, including the postantibiotic effect (PAE), are performed at pH 7.4 in noncationic-supplemented media, a situation which may differ significantly from the true microenvironment in most infected foci. We studied the impact of five different pH levels (pH 5, 6, 7, 7.4, and 8) on the duration of the PAE, the MIC, and bactericidal activity. Acid pH was found to have in general a deleterious effect on the activity of aminoglycosides and ciprofloxacin against Escherichia coli and Pseudomonas aeruginosa, with the MIC being higher, the bactericidal rate being lower, and the PAE being shorter at pH 5 (and to a lesser extent at pH 6) than at more alkaline pH levels. Similar results were observed for imipenem against P. aeruginosa. The PAEs induced by ampicillin against E. coli and dicloxacillin against Staphylococcus aureus were not predictably dependent on the pH, whereas the PAEs induced by ciprofloxacin against S. aureus were longest at either end of the pH spectrum. The bactericidal activity of these agents was, however, pH dependent, being slower at acid pHs. The addition of 50 mg of Ca2' and 20 mg of Mg2+ per liter of liquid medium at pH 7.4 did not affect the duration of the PAE. Since the pH in abscess cavities may be close to 5, these observations may be of importance for employment of the agents studied in closed or poorly drained infections.

mentioning

confidence: 99%

Impact of pH and cationic supplementation on in vitro postantibiotic effect

Guðmundsson¹,

Erlendsdóttir²,

Gottfreðsson³

et al. 1991

“…These include reduced suscep-tibility to lysostaphin (17), deficiency in protein A (23), and a change in the net surface charge of the cells (9). In addition, variation in several growth parameters was found to have profound influence on the expression of methicillin resistance.…”

mentioning

confidence: 99%

Altered penicillin-binding proteins in methicillin-resistant strains of Staphylococcus aureus

Hartman¹,

Tomasz²

1981

153

The penicillin-binding proteins (PBPs) of a methicillin-resistant (MR) and a methicillin-susceptible (MS) Staphylococcus aureus were compared by various approaches involving the use of high-specific-activity [3H]penicillin as a reagent. The MR and MS strains were found to contain PBPs of the same number and electrophoretic mobilities. However, saturation of PBPs 1, 2, and 3 by methicillin in the MR strain required the use of several thousands of micrograms of antibiotic per milliliter, whereas 0.2 to 0.4 ,ug of methicillin per ml was sufficient to effectively compete with [3H]penicillin for the PBPs of the MS strain. Additional experiments indicate that these differences most likely reflect a greatly decreased affinity of the PBPs of the MR strain as compared to those of the MS strain. Shift of the pH of the culture medium of the MR strain from pH 7.0 to 5.2 resulted in an immediate drop in phenotypic resistance to methicillin (from a minimal inhibitory concentration value of 3,200 /Lg/ml at pH 7.0 to 0.8 ,ug/ml at pH 5.2). Examination of the methicillin affinities of PBPs in MR bacteria grown at pH 5.2 showed the presence of the same low-affinity PBPs as in bacteria grown at pH 7.0. Thus, the pH-dependent resensitization to methicillin cannot be explained by a parallel increase in the antibiotic affinities of the PBPs.Extensive studies on the nature of methicillin resistance in Staphylococcus aureus have led to the recognition of several pleimorphic properties in these mutants. These include reduced susceptibility to lysostaphin (17), deficiency in protein A (23), and a change in the net surface charge of the cells (9). In addition, variation in several growth parameters was found to have profound influence on the expression of methicillin resistance. Specifically, growth of resistant mutants at low pH (16,20) or in the presence of ethylenediaminetetraacetate (19) was shown to result in suppression of methicillin resistance. Furthermore, it was reported that, with at least some of the methicillin-resistant isolates, bacterial cultures appeared to be unable to fully express resistance if the culture was passed in drug-free medium and subsequently plated on the surface of agar plates containing methicillin. Addition of certain supplements to the agar such as high concentrations of sucrose or sodium chloride, or incubation of the plates at temperatures suboptimal for growth (30°C instead of 37°C), resulted in the recovery of an increased proportion of the population behaving as phenotypically methicillin-resistant bacteria (1, and for review 16). Serial passage in the presence of beta-lactams had the same effect (18), and the term "heterogeneity" was used to refer to this phenomenon.Biochemical studies on amino acid, amino sugar, and teichoic acid composition (24) failed to detect significant chemical differences between the methicillin-susceptible (MS) and -resistant (MR) staphylococci. These studies have led to an extensive characterization of the physiology of MR staphylococci. Nevertheless, the biochemical bas...

“…Methicillin-resistant strains of Staphylococcus aureus by definition are not inhibited by less than 25 ,ug of methicillin per ml, and some will grow in the presence of > 1,600 ,ug/ml (13), whereas ordinary strains are usually inhibited by 1.6 to 3.1 ,uglml (14). The mechanism for the resistance is not known, but it is not due to antibiotic inactivation and is therefore considered to be "intrinsic."…”

mentioning

confidence: 99%

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

Sabath

Wallace

Gerstein

1972

Self Cite

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 B-lactamase-mediated resistance was only slightly affected by pH; methicillin-resistant staphylococci that are ,B-lactamasenegative are models of pure intrinsic resistance, and the common ,B-lactamase-producing organisms (methicillin-susceptible) are examples of pure f3-lactamasemediated 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 g-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.