Presence of an additional penicillin-binding protein in methicillin-resistant Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus hominis, and Staphylococcus simulans with a low affinity for methicillin, cephalothin, and cefamandole

Occurrence of II and V staphylococcal cassette chromosome mec types among coagulase-negative staphylococci from Northeastern part of India

Introduction

Coagulase Negative Staphylococci, the most commonly isolated pathogen are becoming emerging threats to the community as well as to the nosocomial environment. The present study underscores the distribution of Staphylococcal cassette chromosome mec (SCCmec) types among Methicillin resistant Coagulase Negative Staphylococci from the environmental origin.

Methods and Materials

Environmental and food sample (n = 460) from different location of northeastern region of India were collected for a period of one year and were phenotypically and genotypically screened using cefoxitin disc and PCR techniques for mecA and mecC gene detection. All the MR-CoNS isolates possessing mecA gene were subjected to 16srDNA sequencing for species identification. SCCmec typing was determined by evaluating using primer sets from type I to type V. Antibiotic susceptibility testing was performed for all the isolates. Statistical analysis with chi-square test using SPSS-21 statistical software.

Results

Methicillin resistance shown by one hundred forty three isolates were carried out for molecular analysis, among them 53.84% serves as mecA carrier. Distribution of Staphylococcus haemolyticus was more frequent and was found that SCCmec types II and V were predominant among the study isolates. Linezolid was the drug of choice for the CoNS isolates. Statistical analysis showed an insignificant result for the tested antibiotics and SCCmec types.

Conclusion

This study therefore interprets the relative importance of SCCmec types among MR-CoNS isolates.

Antimicrobial resistance in coagulase-negative staphylococci from Nigerian traditional fermented foods

Background

Coagulase-negative staphylococci have become increasingly recognized as the etiological agent of some infections. A significant characteristic of coagulase-negative staphylococci especially strains isolated from animals and clinical samples is their resistance to routinely used antibiotics although, resistant strains isolated from fermented foods have not been fully reported.

Methods

A total of two hundred and fifty-five CoNS isolates were subjected to antimicrobial susceptibility test using the disc diffusion technique. The minimum inhibitory concentration of the isolates to the tested antibiotics was determined using the microbroth dilution method. Methicillin resistant strains were confirmed by detection of methicillin resistant genes (mecA) and also employing cefoxitin screening test.

Results

The isolates were confirmed to be methicillin resistant by the detection of mecA genes and the cefoxitin screening test. The isolates demonstrated appreciable resistance to ampicillin (86.7%), sulfomethoxazole–trimethoprim (74.9%), amoxicillin–clavulanic acid (52.5%) and oxacillin (35.7%). Methicillin resistance was exhibited by 13 out of the 255 isolates although no mecA gene was detected. It was also observed that the methicillin resistant isolates were prevalent in these traditional foods; iru, kindirmo, nono and wara.

Conclusion

This study has ameliorated the incidence of multiple antibiotic resistant coagulase-negative staphylococci in Nigerian fermented foods and if not tackled adequately might lead to horizontal transfer of antibiotic resistance from food to man.

Detection of Methicillin-Resistant Coagulase-Negative Staphylococci and Surveillance of Antibacterial Resistance in a Multi-Center Study from Iran

BACKGROUND

Coagulase-negative staphylococci (CNS) are a common cause of nosocomial infections. In recent years, an increase in the incidence of methicillin-resistant coagulase-negative staphylococci (MRCNS) has led to the severity of the disease.

OBJECTIVES

The aim of this study was to isolate and identify MRCNS strains by oxacillin disk agar diffusion, oxacillin agar screening, and polymerase chain reaction (PCR) and to evaluate their antibacterial resistance patterns.

PATIENTS AND METHODS

Totally, 122 CNS isolates were collected from the clinical specimens of four hospitals in Iran. Susceptibility testing was performed by disk agar diffusion against 15 antimicrobial agents. Then, disk agar diffusion, agar screening, and PCR were applied to determine susceptibility to oxacillin.

RESULTS

Out of the 122 isolates, 92 isolates were found to be MRCNS by PCR. The sensitivities and specificities of disk agar diffusion and agar screening were 89.2% and 69% and 93.8% and 96.3%, respectively. Also, 93 CNS isolates were resistant to Methicillin according to disk agar diffusion.

DISCUSSION

Our results indicated that agar screening was superior to oxacillin disk agar diffusion. A comparison between the antibiotic sensitivity patterns of the MRCNS and the Methicillin-Susceptible Coagulase-Negative Staphylococci (MSCNS) showed that the MRCNS were predominantly multiple-drug resistant isolates as the simultaneous resistance rate to 4 or more antibiotics in the MRCNS and MSCNS was 93% and 56%, respectively.

Differentiation between Staphylococcus aureus and coagulase-negative Staphylococcus species by real-time PCR including detection of methicillin resistants in comparison to conventional microbiology testing

BACKGROUND

Staphylococcus aureus has long been recognized as a major pathogen. Methicillin-resistant strains of S. aureus (MRSA) and methicillin-resistant strains of S. epidermidis (MRSE) are among the most prevalent multiresistant pathogens worldwide, frequently causing nosocomial and community-acquired infections.

METHODS

In the present pilot study, we tested a polymerase chain reaction (PCR) method to quickly differentiate Staphylococci and identify the mecA gene in a clinical setting.

RESULTS

Compared to the conventional microbiology testing the real-time PCR assay had a higher detection rate for both S. aureus and coagulase-negative Staphylococci (CoNS; 55 vs. 32 for S. aureus and 63 vs. 24 for CoNS). Hands-on time preparing DNA, carrying out the PCR, and evaluating results was less than 5 h.

CONCLUSIONS

The assay is largely automated, easy to adapt, and has been shown to be rapid and reliable. Fast detection and differentiation of S. aureus, CoNS, and the mecA gene by means of this real-time PCR protocol may help expedite therapeutic decision-making and enable earlier adequate antibiotic treatment.

Penicillin-binding proteins and beta-lactam resistance

A number of ways and means have evolved to provide resistance to eubacteria challenged by beta-lactams. This review is focused on pathogens that resist by expressing low-affinity targets for these antibiotics, the penicillin-binding proteins (PBPs). Even within this narrow focus, a great variety of strategies have been uncovered such as the acquisition of an additional low-affinity PBP, the overexpression of an endogenous low-affinity PBP, the alteration of endogenous PBPs by point mutations or homologous recombination or a combination of the above.

Distribution of staphylococcal cassette chromosome mec (SCCmec) types I, II, III and IV in coagulase-negative staphylococci from patients attending a tertiary hospital in southern Brazil

Coagulase-negative staphylococci (CoNS) are now recognized as the aetiological agents of an important range of infections in humans. Most developed countries have reported an increase in CoNS infections in hospitalized patients that are resistant to meticillin and other antibiotics. Staphylococcal cassette chromosome mec (SCCmec) typing is essential for understanding the molecular epidemiology of meticillin-resistant Staphylococcus strains. SCCmec elements are currently classified into types I to VI based on the characteristics of the mec and ccr gene complexes and are further classified into subtypes according to their ‘junkyard DNA’ region. We evaluated the distribution of SCCmec types in CoNS from patients attending the Hospital de Clínicas de Porto Alegre over the period August 2004–December 2005. Among the 129 bloodstream isolates, 36 (27.9 %) harboured SCCmec type I, 4 (3.0 %) harboured SCCmec type II, 67 (52 %) harboured SCCmec type III, 1 (0.8 %) harboured SCCmec type IV and 4 (3.0 %) harboured SCCmec types I and III. Seventeen isolates were not typable. Identification of CoNS at the species level indicated that Staphylococcus epidermidis was the most common species, with 87 isolates, followed by Staphylococcus haemolyticus (15), Staphylococcus hominis (13), Staphylococcus capitis (12) and Staphylococcus sciuri (1). SCCmec type III was the most prevalent among isolates of S. epidermidis (52 %). Among these strains, 30 (23 %) harboured a modified SCCmec type III which contained an additional dcs region in comparison with regular type III. SCCmec type III was also highly prevalent (75 %) among S. capitis isolates. The predominant SCCmec type found among S. haemolyticus isolates was type I. However, all four isolates harbouring SCCmec type II belonged to S. haemolyticus. Our results indicate that SCCmec type III was the most prevalent among the CoNS. Isolates with SCCmec type III were more resistant to non-β-lactam antimicrobials than isolates harbouring SCCmec types I, II and IV, although the increase in resistance was statistically significant only for clindamycin (P=0.021), rifampicin (P=0.010) and levofloxacin (P=0.005).

[Accuracy of four agar diffusion methods and the Vitek 2 automated system for the detection of the methicillin resistance in coagulase negative staphylococci]

AIM OF THE STUDY

Technical difficulties may occur in the detection of methicillin resistance in coagulase negative staphylococci. Phenotypic methods, such as disk diffusion testing, sometimes fail to detect methicillin resistance because of its poor expression. Four of these methods were compared with the VITEK 2 system.

MATERIALS AND METHODS

The accuracy of the VITEK 2 system and the agar diffusion methods according to the current CASFM and NCCLS guidelines were assessed for methicillin susceptibility testing of 70 coagulase negative staphylococci isolates harboring the mecA gene for which the detection of the methicillin resistance was particularly difficult: the CASFM recommendations applied in 1998 failed to categorize them as methicillin-resistant.

RESULTS

Among the different methods currently suggested by the CASFM, those using a heavy inoculum and a hypersaline medium give better results, but they remain however less efficient than the NCCLS recommendations testing (sensitivity: 71-86% vs. 94-94% respectively). The VITEK 2 was more efficient (sensitivity: 91%) than the disk diffusion methods recommended by the CASFM and slightly less efficient than the method recommended by the NCCLS.

CONCLUSION

The VITEK 2 automated system, compared with the disk diffusion methods, showed adequate accuracy for detection of oxacillin resistance in coagulase negative staphylococci.

A survey of antibiotic resistance in Micrococcaceae isolated from Italian dry fermented sausages

The transfer of bacteria that are resistant to antimicrobial agents or resistance genes from animals to humans via the food chain is increasingly a problem. Therefore, it is important to determine the species and the numbers of bacteria involved in this phenomenon. For this purpose, 148 strains of microstaphylococci were isolated from three types of Italian dry fermented sausages. Eight of 148 strains belonged to the genera Kocuria and Micrococcus. The remaining 140 strains belonged to 11 different species of the genus Staphylococcus. The species most frequently isolated was Staphylococcus xylosus, followed by Staphylococcus saprophyticus and Staphylococcus aureus. Antibiotic resistance levels differed among the species and depended on the strain origin. Microstaphylococci were generally susceptible to beta-lactams, but 12 strains were resistant to methicillin, 8 were resistant to oxacillin, and 9 were resistant to penicillin G. No resistance was observed for aminoglicosides and cephalosporines. Many strains were resistant to sulfonamide, colistin suphate, tetracyclin, and bacitracin. Two strains of S. aureus, four strains of S. xylosus, and one strain of Staphylococcus sciuri were able to grow in the presence of 8 microg of vancomycin per g, but all strains were susceptible to teicoplanin. Twenty-two microstaphylococci were resistant to at least five of the tested antibiotics. The multiresistant strain S. aureus 899 was unaffected by eight antibiotics, including vancomycin and methicillin, indicating that a more prudent use of antibiotics in animal husbandry and better hygienic conditions during production should be encouraged because they can play a major role in reducing the incidence of such multiresistant microorganisms and the possible spread of the genetic elements of their resistance.

Isolation of methicillin-resistant coagulase-negative staphylococci from patients undergoing continuous ambulatory peritoneal dialysis (CAPD) and comparison of different molecular techniques for discriminating isolates of Staphylococcus epidermidis

Coagulase-negative staphylococci (CNS) have emerged as an important pathogen in nosocomial infections. About 80%-90% of CNS isolates associated with hospital infections are methicillin-resistant coagulase-negative staphylococci (MRCNS). The aims of this study were to screen for MRCNS isolates in the flora of a small population of patients undergoing continuous ambulatory peritoneal dialysis (CAPD) and to evaluate the discriminatory power of different molecular methods: pulsed-field gel electrophoresis (PFGE), mecA location, ClaI/mecA polymorphism and arbitrarily primed polymerase chain reaction (AP-PCR) for characterizing isolates of methicillin-resistant Staphylococcus epidermidis (MRSE). Seventy-nine CNS isolates were recovered from the 11 CAPD patients studied. Using a methicillin screening agar and a DNA specific mecA probe we verified that 30 of the 79 (38%) CNS isolates were resistant to methicillin (MRCNS). Twenty-two of the 30 MRCNS (73%) were MRSE, 7 (23%) methicillin-resistant S. haemolyticus (MRSH(ae)) and 1 (3%) methicillin-resistant S. hominis (MRSH(om)). All patients analyzed carried MRCNS in their flora, in one or more sites. Since CAPD patients have high risk for developing peritonitis, the colonization of these patients with MRCNS might represent an additional problem, due to the therapeutic restrictions imposed by these multiresistant isolates. A wide genetic diversity was verified when the PFGE of the MRSE isolates was analyzed. The 22 MRSE isolates displayed a total of 15 PFGE different patterns (11 PFGE types and 4 subtypes). The location of mecA in the SmaI-fragmented genome DNA did not bring any additional advantage for epidemiologic characterization of the isolates. The ClaI/mecA polymorphism was able to correctly discriminate 12 from the 15 PFGE patterns. In addition, the DNA of 20 MRSE isolates were used for AP-PCR typing. These isolates belonged to 14 PFGE patterns (11 types and 3 subtypes) and displayed 15 genotypes (for the association of PFGE, mecA location and ClaI/mecA polymorphism). A total of 17 different amplification patterns was verified using the primer 1. Only for 2 genotypes, strains having identical genetic backgrounds were further discriminated by AP-PCR (2 of 15 genotypes (87%) for AP-PCR and 1 of 15 genotypes for PFGE; (93%). Concluding, our results indicated that the AP-PCR can be an alternative and useful tool for monitoring and genotyping MRSE colonization and also to molecular characterizing MRSE outbreaks in hospitals.

Rapid detection of the methicillin-resistance gene, mecA, in coagulase-negative Staphylococci

Phenotypical methods are routinely used to detect methicillin resistance in Staphylococci. These methods are time-consuming and there are difficulties in detecting all resistant strains carrying the mecA gene. We detected methicillin-resistant Staphylococci in biological samples by PCR amplification of mecA, without the time-consuming step of identifying a bacterial isolate. Methicillin-resistant Staphylococci isolates were also detected by screening on agar supplemented with oxacillin. The biological samples were collected from the hands of 17 healthcare workers at the Department of Paediatrics at the University Hospital of Tromsø. mecA was amplified in 12 of the 17 samples. The gene was verified by DNA sequencing of the PCR amplicon. Using the phenotypical method, methicillin-resistant Staphylococci were isolated from 6 of the samples. In all 6 of these samples, mecA was amplified by PCR. We conclude that PCR is a sensitive and specific method for detecting methicillin resistance in Staphylococci. The PCR detection of mecA is rapid, fairly simple and can easily be assimilated into the routines of a clinical microbiological laboratory.

Methicillin-resistant coagulase-negative staphylococci isolated from healthy horses in Japan

OBJECTIVE

To determine patterns of methicillin-resistant staphylococci isolated from apparently healthy horses.

SAMPLE POPULATION

44 horses from 8 riding clubs in Japan.

PROCEDURE

Methicill in-resistant staphylococci were isolated from the skin or nares, using a selective medium containing a beta-(symboric) lactam antibiotic, ceftizoxime. Clonality of isolates was determined by use of pulsed-field gel electrophoresis. Detection of mecA, mecl, and mecR1 genes was accomplished by use of polymerase chain reactions.

RESULT

Of the 44 horses, 13 (29.5%) yielded 15 isolates of methicillin-resistant staphylococci. The 15 isolates were identified as 6 species (Staphylococcus epidermidis, S lentus, S saprophyticus, S xylosus, S sciuri, and S haemolyticus). However, methicillin-resistant S aureus was seldom isolated. Each isolate contained the mecA gene and had a high resistance to beta-lactam antibiotics. Some isolates also were resistant to other antibiotics such as erythromycin and kanamycin.

CONCLUSIONS AND CLINICAL RELEVANCE

Methicillin-resistant coagulase-negative staphylococci that were highly resistant to various antibiotics were isolated from apparently healthy horses in Japan. These organisms must be considered a potential threat to horses and veterinarians who care for them.

When drug inactivation renders the target irrelevant to antibiotic resistance: a case story with beta-lactams

By challenging the efficiency of some of our most useful antimicrobial weapons, bacterial antibiotic resistance is becoming an increasingly worrying clinical problem. A good antibiotic is expected to exhibit a high affinity for its target and to reach it rapidly, while escaping chemical modification by inactivating enzymes and elimination by efflux mechanisms. A study of the behaviour of a beta-lactamase-overproducing mutant of Enterobacter cloacae in the presence of several penicillins and cephalosporins showed that the minimum inhibitory concentration (MIC) values for several compounds were practically independent of the sensitivity of the target penicillin binding protein (PBP), even for poor beta-lactamase substrates. This apparent paradox was explained by analysing the equation that relates the antibiotic concentration in the periplasm to that in the external medium. Indeed, under conditions that are encountered frequently in clinical isolates, the factor characterizing the PBP sensitivity became negligible. The conclusions can be extended to all antibiotics that are sensitive to enzymatic inactivation and efflux mechanisms and must overcome permeability barriers. It would be a grave mistake to neglect these considerations in the design of future antibacterial chemotherapeutic agents.

Isolation of methicillin-resistant coagulase-negative staphylococci from chickens

Methicillin-resistant coagulase-negative staphylococci were isolated from the nares and skin of 1- to 8-week-old healthy chickens in three flocks from a farm. Isolation of methicillin-resistant coagulase-negative staphylococci was positive for 72 (25.7%) of the 280 chickens tested, with the frequency varying from 2.2 to 100% according to flock. A total of 45 appropriate isolates were selected and subjected to identification. Of the 45 methicillin-resistant coagulase-negative staphylococcal isolates selected, 37 were identified as Staphylococcus sciuri, 5 were identified as Staphylococcus epidermidis, and 3 were identified as Staphylococcus saprophyticus. The distribution of the species was different among the flocks. Comparative analysis of the SmaI-digested chromosomal DNA by pulsed-field gel electrophoresis revealed that the isolates could have originated from a single clone of each of S. sciuri and S. saprophyticus and three clones of S. epidermidis. By two methods based on the PCR technique, the mecA gene was detected in all five representative isolates of each methicillin-resistant coagulase-negative staphylococcal clone. The nucleotide sequence of a PCR fragment obtained from an isolate of S. sciuri was completely identical to the corresponding region of mecA genes reported in human methicillin-resistant Staphylococcus aureus isolates and Staphylococcus epidermidis isolates. The representative methicillin-resistant coagulase-negative staphylococcal isolates were resistant to many beta-lactam antibiotics, and some isolates were also resistant to macrolide and aminoglycoside antibiotics. This is the first evidence of the existence of methicillin-resistant coagulase-negative staphylococci from animals possessing the mecA gene.

Comparison of PCR detection of mecA with standard susceptibility testing methods to determine methicillin resistance in coagulase-negative staphylococci

Approximately 75% of coagulase-negative staphylococci are resistant to methicillin, but it is suspected that even more resistance exists that is not detected by standard susceptibility assays. To determine the most accurate assay for measuring resistance, we compared the detection of mecA by PCR with detection by National Committee for Clinical Laboratory Standards methods using oxacillin as the class drug. Strains from 11 species of coagulase-negative staphylococci were selected such that 84% were susceptible by the broth microdilution method. Of 45 mecA-positive strains, 1 strain was unable to express the mecA gene product after induction and was not included in further analyses. For microdilution with 2% NaCl, the disk test without salt, and agar screen containing 4% NaCl plus-6 micrograms of oxacillin per ml, the sensitivities in detecting the 44 mecA-positive strains were 50, 84, and 70%, respectively, at 24 h and 77, 82, and 100%, respectively, at 48 h. The specificities of microdilution, disk, and agar screen in detecting the 97 strains lacking mecA were 100, 89, and 100%, respectively, at 24 h. Only the disk test proved to be less specific at 48 h (81%). Furthermore, for 10 of the mecA-positive strains plus an additional 8 strains subsequently added to the analyses, the MICs were 2 micrograms/ml at 24 h by the broth microdilution method; all 18 strains were positive for mecA by PCR. Thus, an oxacillin MIC of > or = 2 micrograms/ml indicated resistance and is probably a more appropriate breakpoint than the current National Committee for Clinical Laboratory Standards breakpoint of 4 micrograms/ml for coagulase-negative staphylococci. Strains for which MICs are < 2 micrograms/ml may be methicillin resistant and should be verified as susceptible by oxacillin agar screening with incubation for 48 h.

Immunological detection of penicillin-binding protein 2' of methicillin-resistant staphylococci by using monoclonal antibodies prepared from synthetic peptides

Two synthetic peptides 31 and 32 amino acids in length were prepared as deduced from a known amino acid sequence of penicillin-binding protein 2' (PBP2') of methicillin-resistant Staphylococcus aureus. Two monoclonal antibodies were generated from fused cells of myeloma cells and splenic cells of mice immunized with the synthetic peptides. Western blot (immunoblot) analysis demonstrated specific binding of the antibodies to PBP2' of a methicillin-resistant S. aureus strain. An immunoradiometric assay was developed by using these antibodies for simple detection of PBP2'.

Effective treatment of cephalosporin-rifampin combinations against cryptic methicillin-resistant beta-lactamase-producing coagulase-negative staphylococcal experimental endocarditis

The efficacy of cefazolin or cefpirome alone or combined with rifampin was compared with that of vancomycin alone or combined with rifampin in an experimental model of methicillin-resistant, beta-lactamase-producing, coagulase-negative staphylococcal endocarditis. Phenotypically, the mecA gene-positive strain used in vivo did not exhibit methicillin resistance by the agar dilution or disk susceptibility method but was resistant in vitro (oxacillin MIC, 64 micrograms/ml) by the microtiter dilution method with 2% NaCl supplementation. Macrodilution broth susceptibilities of standard inocula failed to demonstrate cross-resistance of staphylococci to cefazolin (MIC, 8 micrograms/ml) or cefpirome (MIC, 4 micrograms/ml). In vivo, vancomycin and cefpirome had similar activities, and both regimens were more effective than was cefazolin alone. While the MIC of rifampin was low (0.031 micrograms/ml), monotherapy with rifampin resulted in a bimodal distribution of outcomes due to the expected emergence of resistant mutants. The results in vitro of time-kill synergy studies using rifampin in combination with cefazolin or cefpirome varied with the antimicrobial concentrations tested and did not reliably predict activities in vivo of rifampin-beta-lactam combination therapies. Cefpirome, but not cefazolin or vancomycin, in combination with rifampin was synergistic in vivo. Cefpirome in combination with rifampin was more effective than was cefazolin in combination with rifampin. Both cephalosporin-rifampin regimens were significantly more effective than was cephalosporin or vancomycin monotherapy and were as effective as vancomycin combined with rifampin. These data support further evaluation of rifampin-beta-lactam combinations as possible alternative therapies to vancomycin-containing regimens for selected methicillin-resistant coagulase-negative staphylococcal infections.

Detection of methicillin-resistant Staphylococcus aureus (MRSA) with antibodies against synthetic peptides derived from penicillin-binding protein 2'

Ten kinds of peptides (21 to 32 amino acids in length) were synthesized based on the reported amino acid sequences of the penicillin-binding protein 2' (PBP2') of methicillin-resistant Staphylococcus aureus (MRSA). Antibodies against these synthetic peptides (SPs) were generated by immunizing rabbits. The antibodies raised against all the peptides except for one reacted to PBP2' of MRSA and to SPs used for immunization but not to any other protein of MRSA or methicillin-susceptible S. aureus (MSSA) tested by ELISA and Western blotting. A sandwich immunoradiometric assay (IRMA) for the detection of PBP2' was developed using these antibodies. The method could detect PBP2' extracted from as few as 3 x 10(4) cells of a clinical MRSA isolate, and a good correlation between cell number and signal radio-count was observed. IRMA was positive for all 51 methicillin-resistant staphylococci isolated from patients, and was negative for all the 28 methicillin-susceptible ones and 19 strains of other bacterial species. IRMA could be a simple and reliable method for MRSA detection in the clinical bacterial laboratory.

Detection of the Staphylococcal mecA gene by chemiluminescent DNA hybridization

A new solution-phase DNA hybridization capture assay for the rapid detection of the mecA gene in clinical isolates of Staphylococcus was compared with multiplex PCR and disk diffusion methods. The assay uses a DNA capture probe immobilized on paramagnetic particles and a second DNA probe labeled with an acridinium ester. Bacteria from 24-h cultures are lysed, and the lysates are hybridized with the DNA probes. After magnetic separation to remove unhybridized labeled probe, the mecA gene is detected by the chemiluminescence of the hybridized probe. Four hundred consecutive staphylococcal isolates were assayed, including 147 S. aureus and 253 coagulase-negative Staphylococcus isolates. Among the S. aureus isolates, 14 of 147 were MecA+ by both the hybridization capture assay and PCR; 133 of 147 were MecA negative by both assays (positive and negative predictive values, 100%). Comparison of disk diffusion results with those obtained by genotypic methods indicated that 14 of 147 S. aureus isolates judged to be resistant were positive by both methods; 119 of 147 were Oxs and negative by both genotypic methods (positive and negative predictive values, 50 and 100%, respectively). The remaining 14 S. aureus isolates were MecA- Oxr; among these, 13 were beta-lactamase hyperproducers. For coagulase-negative staphylococci, 130 of 253 were MecA+ by the hybridization capture assay; 129 of 130 of these isolates were positive by PCR (positive and negative predictive values, 99.2 and 100%, respectively). Comparison with the disk diffusion assay showed that 128 of the coagulase-negative MecA+ isolates were Oxr; 111 of 253 were MecA- and Oxs (positive and negative predictive values, 90.8 and 99.1%, respectively). Thirteen coagulase-negative isolates were MecA-Oxr; among these, three were beta-lactamase hyperproducers. Comparison of the hybridization capture assay results with PCR indicates that the DNA hybridization assay is a sensitive and specific test for the detection of the mecA gene in clinical isolates of Staphylococcus.

Comparative in vitro activities of L-695,256, a novel carbapenem, against gram-positive bacteria

The in vitro activity of a prototype 2-aryl carbapenem, L-695,256, against gram-positive bacteria was examined. All streptococci and oxacillin-susceptible and -resistant staphylococci were inhibited at concentrations of < or = 0.125, < or = 0.125, and 4 micrograms/ml, respectively. The activity of L-695,256 was superior to that of imipenem against other organisms intrinsically resistant to beta-lactams.

Construction of a modified penicillin-binding protein 2a from methicillin-resistant Staphylococcus aureus and purification by immobilized metal affinity chromatography

The mecA-27r gene, which encodes PBP2a-27r, was modified by site-specific mutagenesis, resulting in replacement of the N-terminal membrane anchor with a short chelating peptide (CP-PBP2a-27r). CP-PBP2a-27r retained the same binding affinity for beta-lactam antibiotics as the wild-type enzyme. Approximately 95% pure CP-PBP2a-27r was recovered in a single step by use of chelating-peptide-immobilized metal ion affinity chromatography.

Update on clinical significance of coagulase-negative staphylococci

The clinical significance of coagulase-negative Staphylococcus species (CNS) continues to increase as strategies in medical practice lead to more invasive procedures. Hospitalized patients that are immunocompromised and/or suffering from chronic diseases are the most vulnerable to infection. Since CNS are widespread on the human body and are capable of producing very large populations, distinguishing the etiologic agent(s) from contaminating flora is a serious challenge. For this reason, culture identification should proceed to the species and strain levels. A much stronger case can be made for the identification of a CNS etiologic agent if the same strain is repeatedly isolated from a series of specimens as opposed to the isolation of different strains of one or more species. Strain identity initially can be based on colony morphology, and then one or more molecular approaches can be used to gain information on the genotype. Many of the CNS species are commonly resistant to antibiotics that are being indicated for staphylococcal infections, with the exception of vancomycin. The widespread use of antibiotics in hospitals has provided a reservoir of antibiotic-resistant genes. The main focus on mechanisms of pathogenesis has been with foreign body infections and the role of specific adhesins and slime produced by Staphylococcus epidermidis. Slime can reduce the immune response and opsonophagocytosis, thereby interfering with host defense mechanisms. As we become more aware of the various strategies used by CNS, we will be in a better position to compromise their defense mechanisms and improve treatment.

Methicillin-resistant Staphylococcus aureus disease in a Portuguese hospital: characterization of clonal types by a combination of DNA typing methods

Fifteen pediatric patients as well as the five nursing staff of the Burn Unit of the Hospital D. Estefania in Lisbon, Portugal, were assayed at weekly intervals over a five-month period in order to identify the nature and number of methicillin-resistant Staphylococcus aureus (MRSA) clones associated with colonization and wound infection. Methicillin resistance was confirmed by a mec-specific DNA probe. MRSA isolates were classified into chromosomal types (clones) on the basis of a variety of techniques: (i) ribotyping; (ii) restriction digestion by the endonuclease ClaI followed by Southern hybridization with the mecA-specific DNA probe and (iii) by hybridization with Tn554; and (iv) pulsed-field electrophoresis (PFE) of SmaI digests followed by (v) Southern hybridization with the mecA DNA probe. A sixth, physiological technique (population analysis) was used to define the mode of phenotypic expression of methicillin resistance in each isolate. All isolates carried a single, common polymorph (ClaI type III) of the mecA gene. Hybridization with Tn554 resolved these isolates to two novel patterns (alpha and beta), of which one (Tn554 alpha) was predominant (90%). This pattern could be further resolved to four closely related PFE types (A through D). In contrast, all isolates with the Tn554 beta pattern belonged to an additional, grossly different PFE type E. The Tn554 beta class was also unique in that these bacteria carried the mecA gene in a SmaI fragment smaller (about 170 kb) than that found in the alpha type strains (194 kb). Most isolates (83%) showed a single heterogeneous (population analysis Class 3) mode of resistance expression. The data demonstrate the full capacity of the globally rare (ClaI type III) MRSA clone for colonization and virulence. The results also document the stability of the complex heterogeneous resistance phenotype as well as the stability of the chromosomal types under conditions of in vivo carriage over a period of several months. In a few isolates the same mecA polymorph was present in several, grossly different genetic backgrounds, suggesting horizontal transfer of the mecA gene.

Distribution of mec regulator genes in methicillin-resistant Staphylococcus clinical strains

The distributions of the mec regulator genes mecI and mecR1, which were identified on the chromosome of mecA-carrying Staphylococcus aureus N315, in methicillin-resistant staphylococci isolated in Japan and various countries were studied. Screening by dot blot hybridization by using polymerase chain reaction (PCR)-amplified probes revealed that at least the 5'-end region of the mecR1 gene was present in all strains tested, whereas about 40% of the strains were negative for the mecI gene. The data suggested that these regulator genes were the original components of the additional mec region DNA of methicillin-resistant S. aureus as well as methicillin-resistant, coagulase-negative staphylococci of seven staphylococcal species (S. epidermidis, S. haemolyticus, S. hominis, S. sciuri, S. capitis, S. caprae, and S. warneri). The mecI gene, which presumably codes for the repressor protein of the mecA gene, was found to harbor a point mutation in all six mecI-positive methicillin-resistant S. aureus strains, and their basal level of mecA gene transcription was elevated compared with that of strain N315, which harbors a presumably intact counterpart of the mecI gene. The data suggested that the mecI gene encodes for a strong repressor function on mecA gene transcription and is deleted or mutated in clinical methicillin-resistant S. aureus strains with high levels of resistance to methicillin.

The crisis in antibiotic resistance

The synthesis of large numbers of antibiotics over the past three decades has caused complacency about the threat of bacterial resistance. Bacteria have become resistant to antimicrobial agents as a result of chromosomal changes or the exchange of the exchange of genetic material via plasmids and transposons. Streptococcus pneumoniae, Streptococcus pyogenes, and staphylococci, organisms that cause respiratory and cutaneous infections, and members of the Enterobacteriaceae and Pseudomonas families, organisms that cause diarrhea, urinary infection, and sepsis, are now resistant to virtually all of the older antibiotics. The extensive use of antibiotics in the community and hospitals has fueled this crisis. Mechanisms such as antibiotic control programs, better hygiene, and synthesis of agents with improved antimicrobial activity need to be adopted in order to limit bacterial resistance.

Rapid detection of the mecA gene in methicillin-resistant staphylococci by enzymatic detection of polymerase chain reaction products

In order to identify methicillin-resistant staphylococci from clinical sources with ease and reliability, enzymatic detection of polymerase chain reaction (ED-PCR) was applied. ED-PCR is based on the capture of amplified products via biotin-streptavidin affinity and the detection of an incorporated hapten in amplified products with an enzyme-linked antibody. In order to identify methicillin-resistant staphylococci of all species, a 150-bp fragment of the mecA gene was targeted for ED-PCR. After PCR was performed with a pair of biotin and dinitrophenol 5'-labeled primers, the reaction mixture was applied to a microtiter well precoated with streptavidin. Thereafter, bound PCR products were detected colorimetrically with alkaline phosphatase-conjugated anti-dinitrophenol antibody. The extraction of DNA from staphylococcal cells for PCR was simplified so that it could be performed within one tube. The total assay, including PCR, took less than 3 h. The sensitivity of mecA gene detection ranged from greater than 5 x 10(2) CFU per tube for Staphylococcus aureus to greater than 5 x 10(3) CFU per tube for Staphylococcus epidermidis. Genotyping results obtained by ED-PCR of 161 tested strains from the colonies (97 strains of S. aureus and 64 strains of coagulase-negative staphylococci) were compared with the phenotypic susceptibilities of the strains to oxacillin. The results of ED-PCR showed excellent agreement with the MICs of oxacillin with very few exceptions; only one strain of S. aureus and two strains of coagulase-negative staphylococci were found to possess the mecA gene, which was discrepant with their phenotypes. Fifty-five blood culture samples were also tested by ED-PCR. For staphylococcal isolates in 33 of the cultures, oxacillin MICs were >4 microgram/ml; 31 of the 33 staphylococcal isolates were determined by ED-PCR to be mecA gene positive. These results suggest that ED-PCR can be used with reasonable confidence in the clinical microbiological laboratory.

Construction of a water-soluble form of penicillin-binding protein 2a from a methicillin-resistant Staphylococcus aureus isolate

The mecA gene from methicillin-resistant Staphylococcus aureus 27r, which encodes the membrane-bound penicillin-binding protein 2a (PBP 2a), was cloned, sequenced, and expressed in Escherichia coli. PBP 2a is the major factor that mediates methicillin resistance in staphylococci. The DNA sequence of the mecA gene from strain 27r was greater than 99% identical to the DNA sequence of other S. aureus mecA genes and the mecA gene from Staphylococcus epidermidis. Analysis of the deduced amino acid sequence of PBP 2a from strain 27r revealed a hydrophobic region at the amino terminus that possessed characteristics of an uncleaved signal peptide such as those found in type II integral membrane proteins. Site-specific mutagenesis was used to modify the strain 27r mecA gene to permit removal of the region encoding the putative transmembrane region (amino acids 2 to 22). When it was expressed in E. coli, the modified mecA gene from strain 27r encoded a water-soluble form of PBP 2a that was detectable in the cytoplasm of transformants. The water-soluble form of PBP 2a protein from S. aureus 27r retained the same binding efficiency for beta-lactam antibiotics as the unmodified membrane-bound PBP 2a from S. aureus 27r.

Survey of methicillin-resistant clinical strains of coagulase-negative staphylococci for mecA gene distribution

A total number of 125 methicillin-resistant (MIC, greater than or equal to 16) coagulase-negative Staphylococcus strains isolated in Japan were surveyed for the distribution of the mecA gene, the structural gene for penicillin-binding protein 2', which is the causative genetic element for the intrinsic resistance of methicillin-resistant Staphylococcus aureus. Screening with colony hybridization by using a cloned mecA gene probe revealed that 121 strains (96.8%) belonging to the nine coagulase-negative Staphylococcus species (S. epidermidis, S. haemolyticus, S. saprophyticus, S. sciuri, S. simulans, S. hominis, S. capitis, S. warneri, and S. caprae) carried mecA in their genome, indicating wide distribution of the gene among coagulase-negative Staphylococcus species. Most (93.4%) of the mecA-carrying strains were producers of penicillinase. Four strains, including two S. haemolyticus and two S. saprophyticus strains, did not carry mecA in spite of their resistance to methicillin. One of them was of low-level resistance (MIC, 16), but three of them had moderate- to high-level resistance to methicillin (MIC, 64). Analysis of gel electrophoretic banding patterns of penicillin-binding proteins of these strains showed absence of penicillin-binding protein 2' but some alterations in signal intensities of the other penicillin-binding proteins. The result indicated that about 3% of methicillin-resistant coagulase-negative staphylococci in these hospitals had a resistance mechanism different from that associated with the production of penicillin-binding protein 2', as has been reported in the case of a borderline methicillin-resistant strain of S. aureus.

Identification of methicillin-resistant strains of staphylococci by polymerase chain reaction

A simple and reliable method using a polymerase chain reaction (PCR) was devised to identify methicillin-resistant staphylococci. By using lysates of the strain to be tested as templates and 22-mer oligonucleotides as primers, a 533-bp region of mecA, the structural gene of a low-affinity penicillin-binding protein (PBP 2'), was amplified by PCR and detected by agarose gel electrophoresis. Results obtained by this method were compared with those obtained by broth microdilution MIC determination for 210 and 100 clinical isolates of Staphylococcus aureus and coagulase-negative staphylococci, respectively. Of 99 mecA-negative S. aureus isolates, 100% of the strains were methicillin susceptible and 98% of the strains were oxacillin susceptible. Three strains (3%) of 111 mecA-positive S. aureus isolates exhibited almost the same susceptibility to beta-lactams as the mecA-negative ones and did not produce detectable amounts of PBP 2' despite the presence of the mecA gene. One of them yielded typically methicillin-resistant variants at a low frequency with concomitant recovery of PBP 2' production. The mecA gene was also found in coagulase-negative Staphylococcus epidermidis, Staphylococcus haemolyticus, Staphylococcus sciuri, Staphylococcus saprophyticus, and Staphylococcus caprae and conferred resistance on most of the bacteria.