Methicillin Susceptibility Research Articles

Comparative in vitro activity of lomefloxacin, a difluoro-quinolone

Lomefloxacin is a new difluoro-quinolone. In this study, we have determined the in vitro activity of lomefloxacin against a wide range of clinical bacterial isolates and compared it with that of other fluoro-quinolones and some unrelated antimicrobials. Lomefloxacin was very active against Enterobacteriaceae (MIC 90, 0.5 μg/ml) with activity comparable to that of ofloxacin (MIC 90, 0.25 μg/ml). Lomefloxacin was moderately active against isolates of Pseudomonas aeruginosa (MIC 90, 4 μg/ml), and again the activity was comparable to ofloxacin (MIC 90, 4 μg/ml) but was eightfold less than ciprofloxacin (MIC 90, 0.5 μg/ml). Lomefloxacin was also active against isolates of Staphylococcus aureus (MIC 90, 1 μg/ml), irrespective of methicillin susceptibility, and this activity was most comparable to ofloxacin (MIC 90, 0.5 μg/ml) and ciprofloxacin (MIC 90, 0.5 μg/ml). Lomefloxacin was fourfold less active than either ofloxacin or ciprofloxacin against isolates of Enterococcus faecalis (MIC 90, 8 μg/ml) and Streptococcus pneumoniae (MIC 90, 8 μg/ml). In common with ofloxacin and ciprofloxacin, lomefloxacin was very active against isolates of Neisseria spp. (MIC 90, ⩽0.06 μg/ml), Haemophilus spp. (MIC 90, ⩽0.06 μg/ml), Legionella spp. (MIC 90, ⩽0.06 μg/ml), Vibrio spp. (MIC 90, ⩽0.06 μg/ml), and Campylobacter jejuni (MIC 90, 1 μg/ml). Lomefloxacin showed poor activity against isolates of Bacteroides spp. (MIC 90, 16 μg/ml) or Clostridium difficile MIC 90, 32 μg/ml) and was only moderately active against isolates of Clostridium perfringens (MIC 90, 2 μg/ml), Peptostreptococcus spp. (MIC 90, 4 μg/ml), Chlamydia trachomatis (MIC 90, 4 μg/ml), Mycoplasma hominis (MIC 90, 2 μg/ml), and Ureaplasma urealyticum (MIC 90, 8 μg/ml). Lomefloxacin was found to be bactericidal at concentrations generally close to the MIC with 〉3 log 10 reduction in viability of exponentially dividing cultures of Escherichia coli and S. aureus within 5 hr of exposure to concentrations at eight times the MIC. These results indicate a potential clinical role for lomefloxacin in the treatment of genitourinary tract infections caused by Gram-positive and Gram-negative bacteria, respiratory tract infections caused by susceptible organisms, and soft tissue infections caused by S. aureus.

Rapid detection (4 h) of methicillin-resistant Staphylococcus aureus by a bioluminescence method.

A 4-h bioluminescence method for methicillin susceptibility determination was compared with reference methods. Of the Staphylococcus aureus strains tested, 80 were methicillin resistant, 180 were methicillin susceptible, and 10 were borderline susceptible. There was 100% correlation between bioluminescence and reference methods for methicillin-susceptible and methicillin-resistant strains. All borderline-susceptible strains were identified as methicillin resistant by bioluminescence.

Methicillin susceptibility testing of Staphylococcus aureus on media containing five percent sodium chloride.

Methicillin-resistant strains of Staphylococcus aureus are commonly heterogeneous in their expression of resistance to ~-lactam agents in that wide differences in the degree of resistance to methicillin are seen among individual organisms in a population (1). Expression of resistance is favoured by incubating cultures at lower temperatures (2). Consequently, in routine methicillin susceptibility tests organisms are incubated at 30 ~ in some methods. An alternative to incubation at lower temperatures is an increase in the osmolality of the medium by addition of 5 % NaC1 (3), 20% sucrose (4) or 4% polyvinylpyrrolidone (5). The most common additive is 5% NaC1, although its effect on methicillin susceptibility has been tested largely by methods to determine the MIC in liquid media and seldom by the diffusion methods routinely used in clinical microbiology laboratories. In view of the paucity of data regarding the reliability of diffusion methods for testing the susceptibility of Staphylococcus aureus to methicillin on media containing 5% NaC1, a study of this was undertaken.

Methicillin-resistant Staphylococcus aureus at children's hospitals in the United States.

Although methicillin-resistant Staphylococcus aureus (MRSA) has emerged as an important pathogen in hospitalized adults in the United States, reports of MRSA in pediatric patients have been infrequent. To determine the frequency at which MRSA is isolated from children, we surveyed the directors of microbiology at all acute care children's hospitals in the United States, and 57 of 67 (85%) laboratory directors responded to a mailed questionnaire. Those not testing S. aureus for methicillin susceptibility were excluded from the analysis. Of 53 (57%) laboratory directors 30 reported that MRSA had been isolated from patients in their hospitals. Between 1973 and 1981 the proportion of hospitals isolating MRSA increased significantly; 1 of 53 hospitals reported MRSA in 1973 compared to 20 of 53 hospitals in 1981 (P less than 0.001). Large hospitals (greater than or equal to 200 beds) reported MRSA isolates more frequently than did small hospitals (less than 200 beds) (P = 0.007). No association was found between the isolation of MRSA and the presence of burn or intensive care units, residency training programs or rotation of residents to other hospitals. MRSA isolation varied by standard metropolitan statistical area and geographic region. These data show that the isolation of MRSA is increasing in frequency in pediatric patients and that the reporting of MRSA from children's hospitals varies by hospital size, standard metropolitan statistical area and region. Since MRSA causes significant morbidity and mortality, further studies are necessary to identify the risk factors for MRSA infections and to develop effective control measures.

In-vitro activity of methicillin against clinical isolates of Staphylococcus aureus.

Methicillin activity against 149 penicillin-resistant, methicillin-susceptible Staphylococcus aureus strains from bacteraemia cases with endocarditis (n = 89) or without endocarditis (n = 60), from the years 1976-1981, was studied with broth dilution and agar dilution. While no differences in methicillin susceptibility were found in relation to the origin of the strains, Staph. aureus of the phage type complex 94,96 showed significantly higher MIC and IC50 by agar dilution than strains of other phage groups/complexes. This difference probably has no clinical importance but is of epidemiological interest. Broth dilution MIC was generally one dilution higher than agar dilution MIC, possibly explained by methodological factors. The MBC/MIC ratios never exceeded two in any of the strains, indicating a lack of tolerance in these clinically important isolates.

A rapid bioluminescent method for the detection of methicillin-resistant Staphylococcus aureus colonies.

A rapid method for the detection of methicillin-resistant Staphylococcus aureus is described. A bioluminescent technique was used to compare the amounts of extracellular adenosine triphosphate released by methicillin-resistant and methicillin-sensitive strains. When tested by this method 29 Staph. aureus strains of known methicillin susceptibility were correctly identified.

METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS AT CHILDREN'S HOSPITALS IN THE UNITED STATES

Although methicillin-resistant Staphylococcus aureus (MRSA) have emerged as important pathogens in hospitalized adults in the United States, reports of MRSA in pediatric patients have been infrequent. To determine the prevalence of MRSA in this population, we surveyed all acute-care children's hospitals in the U.S. 57/67 (85%) hospitals with microbiology laboratories responded to a mailed questionnaire. Those not testing S. aureus for methicillin susceptibility were excluded. 30/53 (57%) reported MRSA isolates. Bacteriologic methods were similar at all hospitals. MRSA were reported at 20/53 hospitals in 1982 compared to 1/53 in 1973 (p<0.001). Large hospitals (≥200 beds) were more likely to have MRSA (p=0.02). No association was found between MRSA and the presence of burn or intensive care units, residency training programs, or rotation of residents to other hospitals. The proportion of hospitals with MRSA varied from 5/6 (83%) in the Northeast to 4/11 (36%) in the West. MRSA were reported more frequently from hospitals in large metropolitan areas (8/10 vs 22/43). These data show that MRSA are increasing in their importance as pathogens in the pediatric population.

Determination of susceptibility of Staphylococcus aureus to methicillin by luciferin-luciferase assay of bacterial adenosine triphosphate.

Susceptibility of 50 strains of Staphylococcus aureus to methicillin was determined by disc diffusion, minimum inhibitory concentration (MIC) test and luciferin-luciferase assay of bacterial adenosine triphosphate (ATP). ATP concentration time curves calculated on eight strains (four sensitive, four resistant) incubated at 30 degrees C indicated 2.5 h as the optimum time for determination of methicillin susceptibility. The ATP concentration and relative light unit (RLU) value of each test broth (antibiotic-containing), expressed as a percentage of its own control broth (antibiotic-free) indicated values of less than 30% for ATP and less than 40% for RLU to be indicative of methicillin sensitivity. Single time point ATP assays carried out on the 50 strains after 2.5 h at 30 degrees C correlated exactly with disc diffusion and MIC. Luciferin-luciferase assay of bacterial ATP appeared to be a reliable, rapid technique for determining the susceptibility of Staph. aureus to methicillin.

Interlaboratory variation of antibiograms of methicillin-resistant and methicillin-susceptible Staphylococcus aureus strains with conventional and commercial testing systems.

Laboratory-prepared (conventional) and commercial susceptibility testing systems were compared by using a group of methicillin-resistant (MR) and methicillin-susceptible (MS) strains of Staphylococcus aureus. A group of 25 MR and 15 MS S. aureus strains were coded and tested blindly by disk diffusion, agar dilution, broth microdilution, Sensititre, Micro-Media, Sceptor, API 3600S, MicroScan, Autobac I, and MS-2 systems. All systems were incubated at 35 degrees C and read with either a manual or automated reader at the recommended times. Where applicable, systems were also read at 48 h. Among the conventional assays, the broth and agar dilution methods were comparable, both detecting 88% of the MR strains at 24 h and detecting 92 and 96%, respectively, at 48 h. The disk diffusion method was less efficient, detecting only 36 and 72% at 24 and 48 h, respectively. Detection of cephalothin resistance was low for all systems at both time periods, with agar dilution and disk diffusion being the most and least efficient, respectively. Some variability was also seen with detection of resistance to clindamycin and gentamicin. Among the MS strains, variability among the conventional systems occurred with methicillin, gentamicin, ampicillin, and penicillin. Comparison of the commercial systems with manual readers with the broth microdilution method (reference method) showed that for MR strains, the Sceptor system gave identical results at 24 and 48 h. Sensititre detected 68 and 88% of the MR strains, whereas Micro-Media was least effective detecting 12 and 80% at 24 and 48 h, respectively. None of the commercial systems detected cephalothin resistance well, with only one strain being indicated by the Sceptor and Sensititre systems at 48 h. Slight differences were also seen among the systems with clindamycin and gentamicin. With regard to the MS strains, variability among the systems was seen with methicillin, penicillin, ampicillin, clindamycin, and gentamicin. Among commercial systems with automated readers, the API system detected a greater number of MR strains than did the reference method at 24 and 48 h, 96 and 100%, respectively. The MicroScan method was comparable to the reference method detecting 80 and 88% of the MR strains at both time periods, respectively. Both Autobac I and MS-2 were much less effective in detecting MR strains, noting only 32 and 16%, respectively, at the 3- to 6-h readings. Poor detection of cephalothin resistance among MR strains was evident in all systems. Variability also occurred among the systems with clindamycin, gentamicin, and ampicillin. A single strain of the MR group was reported to be vancomycin resistant by the API system. Among the MS group, the greatest variability was seen with methicillin. Less variability occurred with penicillin, ampicillin, gentamicin, and vancomycin.

In vitro susceptibility patterns of methicillin-resistant and-susceptible Staphylococcus auerues strains in a population of parenteral drug abusers from 1972 to 1981.

Since 1980, infections caused by methicillin-resistant (MR) Staphylococcus aureus have been epidemic among Detroit-area parenteral drug abusers. Because of the increasing importance of this pathogen, in vitro susceptibilities were compared for 39 isolates of MR S. aureus from 1980 to 1981, and for 56 strains of methicillin-susceptible (MS) S. aureus from 1972 to 1981, recovered from drug abusers with community-acquired infections. Agar dilution studies were performed at 35 degrees C, and minimal inhibitory concentrations were determined after incubation for 18 and 48 h. MR S. aureus exhibited cross-resistance to other beta-lactam antibiotics which frequently required 48 h for expression. MR S. aureus isolates were also resistant to tetracycline, clindamycin, tobramycin, and amikacin. All MR S. aureus isolates investigated synthesized an aminoglycoside 4'-nucleotidyltransferase. Emergence of resistance to cefotaxime, tetracycline, and clindamycin was noted among current MS S. aureus isolates. Vancomycin, fusidic acid, trimethoprim/sulfamethoxazole, and rifampin were the most active agents against MR S. aureus and were equally effective against MS S. aureus.

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

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.

Classification and characteristics of coagulase-negative, methicillin-resistant staphylococci.

Sixty-five clinical isolates of coagulase-negative, methicillin-resistant staphylococci have been classified as Staphylococcus epidermidis (63.0%), "phosphatase-negative S. epidermidis" (12.3%), coagulase-negative Staphylococcus aureus (6.2%), Staphylococcus hemolyticus (6.2%), Staphylococcus hominis (3.1%), and Staphylococcus warneri (1.5%). Five of the organisms (7.7%) could not be classified with certainty as currently recognized species. Novobiocin resistance was encountered in eight of the strains, but these were not classified as the accepted novobiocin-resistant staphylococcal species. Some differences in antibiotic resistance patterns to those typical of methicillin-resistant S. aureus were noted in that, although 29 strains were resistant to methicillin, penicillin, sulfamethizole, streptomycin, and tetracycline, the remainder of the strains were sensitive to streptomycin or tetracycline or both. In a majority of the strains (42 of 65), methicillin susceptibility testing by the disk method at 30 or at 37 degrees C in the presence of NaCl did not appear to enhance resistance expression. Most of the strains produced beta-lactamase (EC 3.5.2.6), but none of the 21 strains tested produced enterotoxin B.

Lack of correlation between methicillin resistance and susceptibility to lysostaphin in Staphylococcus aureus.

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