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The article considers the evaluation of effectiveness of application of automated polymerase chain reaction system GeneXpert DX ("Cepheid USA) as compared with bacteriologic method in detection of S. aureus (SA) and meticillin-resistant S. aureus (MRSA) under infection of skin and soft tissues in orthopedic patients. The results of analysis of material from patients got in 2009-2011 using analyzer Vitek-2 to identify microorganisms demonstrated that separation and identification of agent was successful in 70.04% of 2153 examined samples. The representatives of genus of Staphylococcus made up 56% of strains. The percentage of MSRA consisted 29.8% of SA isolates. In 2012, the GeneXpert DX system was used to analyze 50 samples of clinical material. The analysis established full matching of results of detection of SA/MRSA in case of using this method and bacteriological analysis to detect agent. The DNA of SA was detected in 61.5% of patients in whom the application of bacteriological method was unsuccessful in separation and identification of agent. In 25% of patients DNA of MRSA was detected. This occurrence made it possible to begin corresponding therapy and to get clinical effect. The diagnostic using GeneXpert DX system took less than 1.5 hours from moment of availability of sample in laboratory. The application of polymerase chain reaction system GeneXpert DX is an effective additional method to identify SA/MRSA which does not exclude application of bacteriological analysis.

Gene targeting in ES cells was used to substitute control of angiotensin converting enzyme (ACE) expression from the endogenous promoter to the mouse c-fms promoter. The result is an animal model called ACE 10/10 in which ACE is overexpressed by monocytes, macrophages, and other myelomonocytic lineage cells. To study the immune response of these mice to bacterial infection, we challenged them with Listeria monocytogenes or methicillin-resistant Staphylococcus aureus (MRSA). ACE 10/10 mice have a significantly enhanced immune response to both bacteria in vivo and in vitro. For example, 5 days after Listeria infection, the spleen and liver of ACE 10/10 mice had 8.0- and 5.2-fold less bacteria than wild type mice (WT). In a model of MRSA skin infection, ACE 10/10 mice had 50-fold less bacteria than WT mice. Histologic examination showed a prominent infiltrate of ACE-positive mononuclear cells in the skin lesions from ACE 10/10. Increased bacterial resistance in ACE 10/10 is directly due to overexpression of ACE, as it is eliminated by an ACE inhibitor. Critical to increased immunity in ACE 10/10 is the overexpression of iNOS and reactive nitrogen intermediates, as inhibition of iNOS by the inhibitor 1400W eliminated all in vitro and in vivo differences in innate bacterial resistance between ACE 10/10 and WT mice. Increased resistance to MRSA was transferable by bone marrow transplantation. The overexpression of ACE and iNOS by myelomonocytic cells substantially boosts innate immunity and may represent a new means to address serious bacterial infections.

Risk factors for developing postoperative mediastinitis (POM) due to methicillin-resistant Staphylococcus aureus (MRSA) were analyzed in a case-case control study of patients who underwent median sternotomy during the period from 1994 through 2000. Three patient groups were studied. The first consisted of 64 patients with POM due to MRSA; the second consisted of 79 patients with POM due to methicillin-susceptible S. aureus (MSSA); and the third consisted of 80 uninfected control patients. In multivariable analysis, patients who were diabetic (adjusted OR, 2.86; 95% CI, 1.22-6.70), female (OR, 2.70; 95% CI, 1.25-5.88), and >70 years old (OR, 3.43; 95% CI, 1.53-7.71) were more likely to develop POM due to MRSA. In contrast, the only independent risk factor associated with POM due to MSSA was obesity (OR, 2.49; 95% CI, 1.25-4.96). Antimicrobial prophylaxis consisted primarily of cephalosporin antibiotics (administered to 97% of the patients). Changes in perioperative antimicrobial prophylaxis, in addition to other interventions, should be considered for prevention of POM due to MRSA in targeted, high-risk populations.

Aims: To analyse population minimum inhibitory concentrations (MICs) data from clinical strains of Staphylococcus aureus and Pseudomonas aeruginosa for changes over a 10-year period and to look for correlations between the antimicrobials tested., Methods and Results: Data from the MIC study of 256 clinical isolates of Staph. aureus [169 methicillin-sensitive Staph. aureus (MSSA), 87 methicillin-resistant Staph. aureus (MRSA)] and 111 clinical isolates of Ps. aeruginosa against eight antimicrobial biocides and several clinically relevant antibiotics was analysed using anova, Spearman-Rho correlation and principal component analysis. Comparisons suggest that alterations in the mean susceptibility of Staph. aureus to antimicrobial biocides have occurred between 1989 and 2000, but that these changes were mirrored in MSSA and MRSA suggests that methicillin resistance has little to do with these changes. Between 1989 and 2000 a sub-population of MRSA has acquired a higher resistance to biocides, but this has not altered the antibiotic susceptibility of that group. In both Staph. aureus and Ps. aeruginosa several correlations (both positive and negative) between antibiotics and antimicrobial biocides were found., Conclusions: From the analyses of these clinical isolates it is very difficult to support a hypothesis that increased biocide resistance is a cause of increased antibiotic resistance either in Staph. aureus or in Ps. aeruginosa., Significance and Impact of the Study: The observation of negative correlations between antibiotics and biocides may be a useful reason for the continued use of biocides promoting hygiene in the hospital environment.

We performed 2 case-control studies among 108 patients with true coagulase-negative staphylococcal (CoNS) bacteremia diagnosed within 48 h of hospital admission. Seventy-nine patients (73%) with methicillin-resistant (MR) CoNS bacteremia and 29 patients (27%) with methicillin-susceptible (MS) CoNS bacteremia were compared with 79 randomly selected control patients without CoNS bacteremia. The presence of a central venous catheter was an independent risk factor for both MR- and MS-CoNS bacteremia (P<.001). Patients with MR-CoNS bacteremia were more frequently admitted from health care facilities (P=.02), more likely to have had previous MR Staphylococcus aureus infection or colonization (P=.02), and more likely to have received antibiotics in the previous 30 days (P=.02). The probability that bacteremia was caused by an MR strain, rather than an MS strain, was 62% among patients admitted from the community and 84% among patients admitted from health care facilities. This study identified epidemiological characteristics that can be used to distinguish between MR- and MS-CoNS bacteremia and found high rates of methicillin resistance among CoNS isolates recovered from patients admitted from the community, as well as from health care facilities.

Context: A rapid, real-time, duplex, fluorescent molecular beacon probe-based polymerase chain reaction (PCR) assay was recently developed for the detection of methicillin-resistant Staphylococcus aureus., Objective: To describe the development and validation of this unique assay., Design: Prospective laboratory analysis., Setting: Urban health region/centralized diagnostic microbiology laboratory. BACTERIAL STRAINS: One hundred eighty-one previously characterized clinical and American Type Culture Collection isolates, including 50 strains each of methicillin-resistant and methicillin-sensitive S aureus, plus 50 strains of coagulase-negative staphylococci and 31 nonstaphylococcal isolates to ensure assay specificity., Intervention: Assays were performed on purified genomic DNA extracted from growing bacterial colonies. Two sets of oligonucleotide primers were used to specifically amplify the mecA and nuc genes, followed by detection of amplicons using fluorophore-labeled molecular beacon probes. Assays were performed on the Mx4000 Multiplex Quantitative PCR System (Stratagene Inc, La Jolla, Calif)., Main Outcome Measures: (1) Assay sensitivity and specificity, and (2) analytical sensitivity., Results: The assay demonstrated 100% sensitivity and 100% specificity, and accurately characterized isolates as methicillin-resistant S aureus, methicillin-sensitive S aureus, or methicillin-resistant coagulase-negative staphylococci, with test results available in 2.5 hours. The analytical sensitivity of the assay was determined to be between 6 and 60 genomic equivalents., Conclusions: This assay is rapid, accurate, easy to perform, and is compatible with other real-time PCR instruments, making it a suitable alternative to conventional PCR methodologies.

Analysis of the antibiotic binding capacity of penicillin-binding proteins (PBPs) in a group of clinical isolates of Staphylococcus epidermidis suggests that the increased level of resistance to methicillin (MIC 4.0 microg/ml) in an isolate free of the mecA gene is due to the decreased binding capacity of PBP3.

Penicillin-binding protein 2a (PBP2a) is the primary beta-lactam resistance determinant of methicillin-resistant Staphylococcus aureus (MRSA). MecA, the gene coding for PBP2a, was cloned with the membrane-anchoring region at the N-terminus deleted. The truncated protein (PBP2a) was overexpressed in Escherichia coli mostly in the soluble form accounting for approximately 25% of soluble cell protein and was purified to homogeneity. The purified protein was shown to covalently bind beta-lactams in an 1:1 ratio as determined by electrospray mass spectrometry. A novel method based on HPLC-elctrospray mass spectrometry has been developed to quantitatively determine the formation of the covalent adducts or acyl-PBP2a complexes. By using this method, combined with kinetic techniques including quench flow, we have extensively characterized the interactions between PBP2a and three beta-lactams and determined related kinetic parameters for the first time. The apparent first-order rate constants (ka) of PBP2a acylation by benzylpenicillin showed a hyperbolic dependence on the concentration of benzylpenicillin. This is consistent with the mechanism that the binding of the penicillin to PBP2a consists of reversible formation of a Michaelis complex followed by formation of the penicilloyl-PBP2a adduct, and allowed the determination of the individual kinetic parameters for these two steps, the dissociation constant Kd of 13.3 mM and the first-order rate constant k2 of 0.22 s-1. From these values, the second-order rate constant k2/Kd, the value reflecting the overall binding efficiency of a beta-lactam, of 16.5 M-1 s-1 was obtained. The fairly high Kd value indicates that benzylpenicillin fits rather poorly into the protein active site. Similar studies on the interaction between PBP2a and methicillin revealed k2 of 0.0083 s-1 and Kd of 16.9 mM, resulting in an even smaller k2/Kd value of 0.49 M-1 s-1. The rate constants k3 for deacylation of the acyl-PBP2a complexes, the third step in the interactions, were measured to be <1.5 x 10(-)5 s-1. These results indicate that the resistance of PBP2a to penicillin inactivation is mainly due to the extremely low penicillin acylating rate in addition to the low association affinity, but not to a fast rate of deacylation. Acylation of PBP2a by a high-affinity cephalosporin, Compound 1, also followed a saturation curve of ka versus the compound concentration, from which k2 = 0.39 s-1, Kd = 0.22 mM, and k2/Kd = 1750 M-1 s-1 were obtained. The 100-fold increase in the k2/Kd value as compared with that of benzylpenicillin is mostly attributable to the decreased (60-fold) Kd, indicating that the cephalosporin fits much better to the binding pocket of the protein.

Time-resolved infrared difference spectroscopy has been used to show that the carbonyl group of the acylenzyme reaction intermediate in the Citrobacter freundii beta-lactamase-catalyzed hydrolysis of methicillin can assume at least four conformations. A single-turnover experiment shows that all four conformations decline during deacylation with essentially the same rate constant. The conformers are thus in exchange on the reaction time scale, assuming that deacylation takes place only from the conformation which is most strongly hydrogen bonded or from a more minor species not visible in these experiments. All conformers have the same (10 cm-1) narrow bandwidth compared with a model ethyl ester in deuterium oxide (37 cm-1) which shows that all conformers are well ordered relative to free solution. The polarity of the carbonyl group environment in the conformers varies from 'ether-like' to strongly hydrogen bonding (20 kJ/mol), presumably in the oxyanion hole of the enzyme. From the absorption intensities, it is estimated that the conformers are populated approximately proportional to the hydrogen bonding strength at the carbonyl oxygen. A change in the difference spectrum at 1628 cm-1 consistent with a perturbation (relaxation) of protein beta-sheet occurs slightly faster than deacylation. Consideration of chemical model reactions strongly suggests that neither enamine nor imine formation in the acyl group is a plausible explanation of the change seen at 1628 cm-1. A turnover reaction supports the above conclusions and shows that the conformational relaxation occurs as the substrate is exhausted and the acylenzymes decline. The observation of multiple conformers is discussed in relation to the poor specificity of methicillin as a substrate of this beta-lactamase and in terms of X-ray crystallographic structures of acylenzymes where multiple forms are not apparently observed (or modeled). Infrared spectroscopy has shown itself to be a useful method for assessment of the uniqueness of enzyme-substrate interactions in physiological turnover conditions as well as for determination of ordering, hydrogen bonding, and protein perturbation.

The presence of an additional penicillin-binding protein (PBP) was demonstrated in methicillin-resistant strains of Staphylococcus epidermidis, S. haemolyticus, S. hominis, and S. simulans. In these four species, the apparent molecular mass of this protein was analogous to that of PBP 2' of methicillin-resistant S. aureus SR 1550-9. It exhibited a low affinity for methicillin, cephalothin, and cefamandole; and its synthesis was methicillin inducible. Peptide mapping of this PBP from the four species yielded identical results that were analogous to those obtained with S. aureus SR 1550-9. These results suggest that this protein is similar to, if not the same as, PBP 2' of S. aureus and that it is involved in methicillin resistance in the four species studied.

We evaluated the distribution of methicillin into normal and damaged heart valves and the accuracy with which subcutaneous plastic chambers reflected tissue uptake of this antibiotic. A total of 24 male New Zealand rabbits were given constant infusion doses of methicillin through central venous catheters. Five of these animals had their aortic and mitral valves damaged by catheterization of the left ventricle. A total of 19 rabbits had perforated plastic chambers inserted subcutaneously 7 to 10 days before methicillin infusion. In all animals more than 80% of the total infused dose of methicillin was accounted for in the serum, urine, and tissues. In the 12 animals infused to steady state (less than 7 h), the steady-state serum concentrations (11 to 120 micrograms/ml) were equal to the concentrations attained in either peritoneal or tissue chamber fluids. In the 12 animals sacrificed before 7 h, tissue chamber concentrations lagged behind serum and heart tissue concentrations in attaining steady state. Steady-state concentrations in normal heart valves and heart muscles failed to increase proportionally to increased constant infusion doses (8.7 to 87.2 mg/kg per h). The steady-state methicillin concentrations in fibrin-scarred heart valves were invariably higher than the steady-state concentrations in the normal right heart of the same animals (P less than 0.05). Tissue uptake of methicillin was altered in scarred heart valves as compared to normal heart valves, and large-volume subcutaneous tissue chambers misrepresented the uptake rate of methicillin into heart tissues and valves.

The antibiotic level in the serum, pericardial fluid and myocardium of 11 patients prepared for heart surgery was tested by means of the agar-diffusion method after the intramuscular administration of 1 g of methicillin. The mean level after 1 to 3 hours was 14.7 mcg/ml in serum, 1.7 mcg/ml in the pericardial fluid and 8.2 mcg/g in the myocardium. In the case of drug sensitivity, the myocardial methicillin concentration is sufficient for the treatment of Gram-positive infections. If the infection is localized in the pericardium, higher doses, local applications, drug combination or complex therapy might be called for.

Methicillin resistance in Staphylococcus aureus has been associated with alterations in the penicillin-binding proteins (PBPs). An intriguing property of all methicillin-resistant staphylococci is the dependence of resistance on the pH value of the growth medium. Growth of such bacteria at pH 5.2 completely suppressed the expression of methicillin resistance. We have examined the PBP patterns of methicillin-resistant staphylococci grown at pH 7.0. We detected a high-molecular-weight PBP (PBP-2a; approximate size, 78,000 daltons) that was only present in the resistant bacteria but not in the isogenic sensitive strain. In cultures grown at pH 5.2, the extra PBP was not detectable.

Probenecid was given intravenously to rabbits in an effort to extend the intraocular longevity of intravitreal methicillin. With relatively high levels of probenecid, an insignificant elevation of half-life was obtained. Aqueous levels of methicillin were measured after intravitreal injection and were found to be adequate to inhibit penicillinase-producing S. aureus for approximately 20 hours.

Mechanisms of absorption, distribution and excretion of methicillin, oxacillin and cephaloridin were studied in 46 women with postnatal mastitis (serous and infiltrative forms). The results obtained allow one to document different clinical efficiency of antibiotics in treatment of patients with postnatal mastitis. Poor efficiency of cephaloridin in therapy of mastitis was shown.

The disposition of methicillin in normal subjects and in subjects with cystic fibrosis (CF) was studied after administration of single intravenous doses of 15 mg/kg. The area under the serum concentration vs. time curve for CF patients was, on the average, only 75% of that found for normal subjects. The low concentrations in serum were caused by more rapid urinary excretion of the antibiotic, with rates of renal clearance averaging 425 ml/min per 1.73 m2 in the patients with CF and 362 ml/min per 1.73 m2 in the normal subjects. No differences were found in volumes of distribution and metabolic clearance rates of methicillin or in rates of creatinine clearance between the two groups of subjects. These data support previous findings with dicloxacillin which show that patients with CF exhibit unusually rapid, active tubular secretion of certain penicillins that may necessitate use of larger doses of these drugs in treatment of infections.

Clindamycin phosphate or methicillin was given preoperatively to 47 patients undergoing hip operations for arthritis. Serum, cortical and cancellous bone specimens were simultaneously obtained during the operation and assayed for antibiotic concentrations. Both gave levels in bone/serum concentration ratio significantly higher for clindamycin than methicillin. No patients studied developed deep infection. It is concluded that clindamycin phosphate and methicillin give comparable concentrations in bone which exceed the minimal inhibitory antibiotic concentration of gram-positive pathogens that frequently cause infection after orthopedic surgical procedures.

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 micrograms of methicillin per ml was sufficient to effectively compete with [3H]penicillin for the PBPs for 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 micrograms/ml at pH 7.0 to 0.8 microgram/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.

The majority of multiresistant Staphylococcus aureus strains isolated in Australian hospitals since 1970 carry a chromosomally-encoded minocycline and tetracycline resistance determinant. By using DNA-DNA hybridization, some of these multiresistant strains were shown to possess also a tetracycline resistance plasmid, equivalent to pT181, integrated into a unique site in the chromosome. By relating the hybridization data to the map of pT181, the site of integration on this plasmid was established to be between the genes for replication and tetracycline resistance.

Features of absorption, distribution and excretion of methicillin, oxacillin and cephaloridine in 58 puerpera were studied. No complications were observed in the postnatal period of the puerpera. The results of the study of the antibiotic pharmacokinetics provided elaboration of the optimal regimen for the use of methicillin, oxacillin and cephaloridine in puerpera, i. e. I g intramuscularly every 6 hours.

An outbreak of methicillin sodium-resistant Staphylococcus aureus (MRSA) infection and colonization, mainly centered in the vascular surgery service, occurred in a 1000-bed tertiary care center between December 1983 and December 1984. Methicillin-resistant S aureus isolated before and during the outbreak was studied by both bacteriophage typing and by restriction endonuclease digestion of bacterial plasmid DNA. Bacteriophage typing was discrepant in nine (56%) of the 17 repeated analyses compared with one (3.4%) of the 29 for plasmid profiling. These typing methods revealed that the epidemic strain was introduced to the hospital from the community 15 months before the outbreak. The outbreak was caused by cross-transmission of the epidemic strain by health care personnel and was controlled by treatment of colonized personnel, education of personnel, and institution of barrier precautions for colonized or infected patients. Plasmid profiling with restriction endonuclease digestion was easier, more rapid, and more specific than bacteriophage typing in the evaluation of this outbreak.

Single-dose and steady-state studies were carried out in rabbits to characterize the pharmacokinetics of methicillin elimination and tissue distribution. Serum methicillin concentrations after single doses exhibited a biexponential decline with a mean terminal half-life of 27 +/- 5 min. Steady-state volume of distribution (Vdss) decreased twofold as doses increased from 5 to 125 mg/kg, while total clearance was consistent over this dosage range. During steady-state infusions of 8.7-87.2 mg/kg/h, the serum and extravascular fluid concentrations increased in proportion to dose, while clearance remained constant. Methicillin tissue concentrations did not increase in proportion to dose and serum concentration, and the Vdss measured from tissue recovery decreased as dosage and serum concentrations increased. Central compartment volume and serum protein binding did not change. The cause of the dose-dependent change in Vdss was a capacity-limited uptake of methicillin into essentially all nonexcretory tissues. This process was described by a partial physiological pharmacokinetic model based on the actual weights or volumes of the rabbit tissues. Distribution into extracellular fluids was assumed to be complete, while entry into cellular, nonexcretory tissue became capacity limited as the Km (17.2 micrograms/mL) was exceeded. The result was a decline in Vdss with increasing serum concentrations to a limiting value which approached the volume of the central space (Vc) and was similar to the extracellular water volume of the rabbit (0.17 L/kg). In the dosage range where total clearance was independent of serum concentration, the distribution of methicillin was concentration dependent and could be predicted by equations derived from the Michaelis-Menten equation.

The penetration of cefazolin, erythromycin and methicillin into normal bone was studied in 20 patients undergoing surgery for fracture in the trochanteric region of the femur. The antibiotic concentrations were determined in serum, bone marrow, and cancellous and cortical bone. For all three antibiotics the bone marrow concentrations were of the same order of magnitude as the serum concentrations. In the eight patients receiving erythromycin, detectable concentrations were found in all the cancellous bone specimens (ranging from 1/7 to 1/2 of the serum concentration) and in three cortical bone specimens (ranging from 1/50 to 1/5 of the serum concentration). In the six patients receiving cefazolin, a detectable concentration was found in only one cancellous bone sample. In the six patients receiving methicillin, detectable concentrations were found only in the blood contaminated specimens of one cancellous and two cortical bone samples. However, by the method used, the recoveries of standard solutions of methicillin in cancellous and cortical bone were about 50 per cent and 15 per cent, respectively.

The penetrations of methicillin, oxacillin, and cephalothin into cortical bone and synovial tissues were studied 1 h after their intravenous administration in 105 patients having arthroplasty of the hip. Although the lowest serum levels were noted with cephalothin (P < 0.01), more patients receiving cephalothin achieved osseous drug levels inhibitory to staphylococci (P < 0.01). Differences in the penetration of the three agents into synovial tissues were not statistically significant.

Methicillin concentrations were measured in serum, normal heart valves, damaged heart valves, myocardium, and extravascular fluid from 12 New Zealand white rabbits to assess the influence of valvular damage on methicillin penetrations. Fibrin scarring of the aortic valve was induced by the placement of a polyethylene catheter through the aortic leaflet for 4 days. Each rabbit was then given a 40-mg/kg intravenous bolus dose of methicillin. Serum concentrations were collected, and animals were sacrificed 5, 15, 30, and 60 min after the dose. Normal and damaged heart valves from six different rabbits were desiccated to evaluate the fluid content of each. The time course of methicillin in damaged aortic valves was similar to that in serum and followed a bioexponential decline. The pharmacokinetic profile of methicillin in normal heart muscle and normal heart valves was clearly different from that of serum and damaged heart valves. Damaged valves showed a rapid and complete equilibrium with serum, whereas normal heart valve and muscle methicillin concentrations were consistently lower than serum concentrations at all times after the rapid bolus dose. The greater extravascular fluid content in damaged heart valves (P less than 0.001) compared with that in normal heart valves may be associated with the greater extent of penetration into damaged heart valves. Equilibrium between serum and damaged valves may be achieved more rapidly because the damaged area is composed of platelet and fibrin matrix and lacks the membrane integrity of normal heart valve tissue.

Methicillin-resistant Staphylococcus aureus (MRSA) has become a significant cause of nosocomial infections. In efforts to delineate the magnitude of this problem, we determined the prevalence of MRSA in community acquired (n = 382) and nosocomial strains (n = 207) of S. aureus isolated between Jan 1986 and March 1989. Antimicrobial susceptibility was evaluated using an agar dilution method (Muller-Hinton agar supplemented with 4% NaCl incubated to 35 degrees C for 24 h) and MIC breakpoints were determined according to NCCLS standards. We detected (24.2%) MRSA in nosocomial strains and (5%) MRSA in community acquired strains (P less than 0.05), with a global prevalence of 11.7%. The susceptibility of community acquired S. aureus was 90% or higher for dicloxacillin, cephalothin, sulbactam/ampicillin (S/A), clindamycin, rifampicin and amikacin; 85% for cefotaxime and SMX/TMP and only 75% for erythromycin. The susceptibility pattern of the nosocomial strains was consistent with the prevalence of MRSA but the susceptibility for cephalothin, amikacin and sulbactam/ampicillin was 84.4%, 89.4% and 86.5% respectively, significantly higher than for methicillin (P less than 0.05). Although the increased susceptibility for cephalothin and amikacin has been reported for MRSA before, the published reports using these antibiotics in the treatment of MRSA infections are controversial. The increased susceptibility of MRSA to S/A could be explained in part if the MR was mediated by "acquired MR" attributable to B-lactamase production. Our data provide a perspective on the magnitude of MRSA as a problem in a pediatric teaching hospital in Mexico. Moreover, if taken at face value, the in vitro susceptibility data point to various potential treatment options which warrant clinical evaluation.

1. The rate of hydrolysis of methicillin, cloxacillin and quinacillin by staphylococcal extracellular penicillinase decreases progressively with time. 2. The inactivation is prevented but not reversed by benzylpenicillin. 3. The rate of inactivation produced by quinacillin is minimal when the rate of hydrolysis is at a maximum. 4. Under certain conditions, partially inactivated enzyme can be reactivated. 5. Combination of the enzyme with antiserum, while permitting hydrolysis, prevents inactivation. 6. No evidence for a stable enzyme-substrate complex has been found.

Pediococcus cerevisiae and P. cerevisiae CRD synthesized peptidoglycan when incubated in a wall synthesis solution inadequate to allow growth.