ermgenes in erythromycin-resistantStaphylococcus aureusand coagulase-negative staphylococci

Staph wars: the antibiotic pipeline strikes back

Antibiotic chemotherapy is widely regarded as one of the most significant medical advancements in history. However, the continued misuse of antibiotics has contributed to the rapid rise of antimicrobial resistance (AMR) globally. Staphylococcus aureus, a major human pathogen, has become synonymous with multidrug resistance and is a leading antimicrobial-resistant pathogen causing significant morbidity and mortality worldwide. This review focuses on (1) the targets of current anti-staphylococcal antibiotics and the specific mechanisms that confirm resistance; (2) an in-depth analysis of recently licensed antibiotics approved for the treatment of S. aureus infections; and (3) an examination of the pre-clinical pipeline of anti-staphylococcal compounds. In addition, we examine the molecular mechanism of action of novel antimicrobials and derivatives of existing classes of antibiotics, collate data on the emergence of resistance to new compounds and provide an overview of key data from clinical trials evaluating anti-staphylococcal compounds. We present several successful cases in the development of alternative forms of existing antibiotics that have activity against multidrug-resistant S. aureus. Pre-clinical antimicrobials show promise, but more focus and funding are required to develop novel classes of compounds that can curtail the spread of and sustainably control antimicrobial-resistant S. aureus infections.

Antibiotics and Bacterial Resistance-A Short Story of an Endless Arms Race

Despite the undisputed development of medicine, antibiotics still serve as first-choice drugs for patients with infectious disorders. The widespread use of antibiotics results from a wide spectrum of their actions encompassing mechanisms responsible for: the inhibition of bacterial cell wall biosynthesis, the disruption of cell membrane integrity, the suppression of nucleic acids and/or proteins synthesis, as well as disturbances of metabolic processes. However, the widespread availability of antibiotics, accompanied by their overprescription, acts as a double-edged sword, since the overuse and/or misuse of antibiotics leads to a growing number of multidrug-resistant microbes. This, in turn, has recently emerged as a global public health challenge facing both clinicians and their patients. In addition to intrinsic resistance, bacteria can acquire resistance to particular antimicrobial agents through the transfer of genetic material conferring resistance. Amongst the most common bacterial resistance strategies are: drug target site changes, increased cell wall permeability to antibiotics, antibiotic inactivation, and efflux pumps. A better understanding of the interplay between the mechanisms of antibiotic actions and bacterial defense strategies against particular antimicrobial agents is crucial for developing new drugs or drug combinations. Herein, we provide a brief overview of the current nanomedicine-based strategies that aim to improve the efficacy of antibiotics.

Evaluation of virulence potential of methicillin-sensitive and methicillin-resistant Staphylococcus aureus isolates from a German refugee cohort

BACKGROUND

Community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) seem to be highly transmissible, often infect otherwise healthy humans and frequently occur in hospital outbreaks.

METHODS

Refugees, living in accommodations in Germany were screened for nasal carriage of S. aureus. The isolates were investigated regarding resistance and virulence, phenotypically and by whole genome data analysis.

RESULTS

5.6% (9/161) of the refugees are carriers of S. aureus. 2.5% (4/161) are MRSA carriers. Among the refugees, spa-types t021, t084, t304, t991 and t4983 were detected, as well as the new spa-types t18794 and t18795. t304 and t991 are assumed to be local spa-types from the middle east. The isolates are less resistant and marginal biofilm formers. Each isolate has a remarkable set of virulence genes, although genes, encoding for proteins strongly associated with invasive S. aureus infections, like Panton-Valentine leucocidin, were not detected.

CONCLUSION

The detection of strains from the middle east, supports the assumption that strains co-travel with the refugees and persist despite a transition of the host's living conditions. Whole genome data analysis does not permit to finally evaluate a germ's virulence. Nevertheless, an impression of the virulence potential of the strains, regarding skills in colonization, resistance, immune evasion, and host cell damaging can be pictured.

Insights into the epidemiology of methicillin-resistant coagulase-negative Staphylococci carriage in community-based drug users

BACKGROUND

Coagulase-negative Staphylococci (CoNS) are opportunistic pathogens. Methicillin-resistant CoNS (MRCoNS) remain a major cause of nosocomial infections, but limited information is available in communities. We, therefore, aimed to investigate the epidemiology of CoNS nasal carriage, especially MRCoNS in community-based drug users.

METHODS

Drug users were included in the cross-sectional study from three communities in Guangzhou, China. A face-to-face questionnaire was used to collect individual information and nasal swabs were collected to identify CoNS and MRCoNS isolates. Phenotypic and genotypic characterization of MRCoNS isolates were detected by using the disk diffusion method and polymerase chain reaction assays. Factors impacting MRCoNS carriage were assessed and estimated odds ratios (ORs) and 95% confidence intervals (CIs) by using logistic regression models.

RESULTS

Overall, 353 drug users were eligible and included in the study. The prevalence of CoNS and MRCoNS were 42.78% and 29.18%, respectively. Having a history of being in prison (aOR = 2.16, 95% CI: 1.10-4.22) and a history of heroin snorting in the past 6 months (aOR = 1.87, 95% CI: 1.14-3.05) were risk factors for MRCoNS nasal carriage in drug users. The proportions of multidrug resistance for CoNS and MRCoNS isolates were respectively 54.38% and 66.07%. The predominant multidrug resistance pattern for CoNS and MRCoNS isolates was simultaneously non-susceptible to penicillin, cefoxitin and trimethoprim-sulfamethoxazole. MRCoNS isolates that were positive for erythromycin- (χ² = 15.49, P < 0.001), tetracycline- (χ² = 44.88, P < 0.001), penicillin- (χ² = 3.86, P = 0.049), clindamycin- (χ² = 4.18, P = 0.028), and gentamicin- (χ² = 20.66, P < 0.001) resistance genes had significantly higher rates of resistance to corresponding antibiotics.

CONCLUSIONS

The prevalence of MRCoNS nasal carriage was high in community-based drug users. Drug users with risk factors should be paid greater attention to. The use of antibiotics in drug users needs further investigation and control.

Two genes involved in clindamycin resistance of Bacillus licheniformis and Bacillus paralicheniformis identified by comparative genomic analysis

We evaluated the minimum inhibitory concentrations of clindamycin and erythromycin toward 98 Bacillus licheniformis strains isolated from several types of fermented soybean foods manufactured in several districts of Korea. First, based on recent taxonomic standards for bacteria, the 98 strains were separated into 74 B. licheniformis strains and 24 B. paralicheniformis strains. Both species exhibited profiles of erythromycin resistance as an acquired characteristic. B. licheniformis strains exhibited acquired clindamycin resistance, while B. paralicheniformis strains showed unimodal clindamycin resistance, indicating an intrinsic characteristic. Comparative genomic analysis of five strains showing three different patterns of clindamycin and erythromycin resistance identified 23S rRNA (adenine 2058-N6)-dimethyltransferase gene ermC and spermidine acetyltransferase gene speG as candidates potentially involved in clindamycin resistance. Functional analysis of these genes using B. subtilis as a host showed that ermC contributes to cross-resistance to clindamycin and erythromycin, and speG confers resistance to clindamycin. ermC is located in the chromosomes of strains showing clindamycin and erythromycin resistance and no transposable element was identified in its flanking regions. The acquisition of ermC might be attributable to a homologous recombination. speG was identified in not only the five genome-analyzed strains but also eight strains randomly selected from the 98 test strains, and deletions in the structural gene or putative promoter region caused clindamycin sensitivity, which supports the finding that the clindamycin resistance of Bacillus species is an intrinsic property.

Differences in distribution of MLS antibiotics resistance genes in clinical isolates of staphylococci belonging to species: S. epidermidis, S. hominis, S. haemolyticus, S. simulans and S. warneri

Background

Macrolides and lincosamides are two leading types of antibiotics commonly used in therapies. The study examines the differences in resistance to these antibiotics and their molecular bases in S. epidermidis as well as in rarely isolated species of coagulase-negative staphylococci such as S. hominis, S. haemolyticus, S. warneri and S. simulans. The isolates were tested for the presence of the erm(A), erm(B), erm(C), lnu(A), msr(A), msr(B), mph(C), ere(A) and ere(B) genes. Phenotypic resistance to methicillin and mecA presence were also determined.

Results

The MLS_B resistance mechanism was phenotypically found in isolates of species included in the study. The most prevalent MLS_B resistance mechanism was observed in S. hominis, S. haemolyticus and S. epidermidis isolates mainly of the MLS_B resistance constitutive type. Macrolide, lincosamide and streptogramin B resistance genes were rarely detected in isolates individually. The erm(B), ere(A) and ere(B) genes were not found in any of the strains. The erm(A) gene was determined only in four strains of S. epidermidis and S. hominis while lnu(A) was seen in eight strains (mainly in S. hominis). The erm(C) gene was present in most of S. epidermidis strains and predominant in S. hominis and S. simulans isolates. The examined species clearly differed between one another in the repertoire of accumulated genes.

Conclusions

The presence of genes encoding the MLS_B resistance among CoNS strains demonstrates these genes’ widespread prevalence and accumulation in opportunistic pathogens that might become gene reservoir for bacteria with superior pathogenic potential.

Biofilm formation in erythromycin-resistant Staphylococcus aureus and the relationship with antimicrobial susceptibility and molecular characteristics

PURPOSE

In this study, we aimed to investigate biofilm formation characteristics in clinical Staphylococcus aureus (S. aureus) isolates with erythromycin (ERY) resistance from China and further analyze their correlations with antimicrobial susceptibility and molecular characteristics.

METHODOLOGY

A total of 276 clinical isolates of ERY-resistant S. aureus, including 142 methicillin-resistant S. aureus (MRSA) strains and 134 methicillin-susceptible S. aureus (MSSA) strains, were retrospectively collected in China. Biofilms were determined by crystal violet staining and ERY resistance genes (ermA, ermB and ermC) were detected by polymerase chain reaction. Inducible clindamycin resistance was examined by D test and multilocus sequence typing, and clonal complexes (CCs) based on housekeeping genes were further determined.

RESULTS

The frequency of biofilm formation among ERY-resistant S. aureus was 40.9% (113/276) in total and no significant difference was found for the frequency of biofilm formation between ERY-resistant MRSA and ERY-resistant MSSA (44.4% vs 37.3%, P > 0.05). In ERY-resistant MRSA isolates, the frequency of biofilm formation in ermA-positive, gentamicin-resistant and ciprofloxacin-resistant isolates was higher than that in ermA-negative, gentamicin-sensitive and ciprofloxacin-sensitive isolates, respectively (63.9% vs 23.6%, P < 0.01; 60.3% vs 27.5%, P < 0.01; 65.2% vs 26.3%, P < 0.01). In addition, tetracycline resistance facilitated biofilm formation in both ERY-resistant MRSA and MSSA and the frequency of biofilm formation in CC239- or CC7S. aureus isolates with ERY resistance was significantly higher compared with that in CC59S. aureus (both P < 0.01).

CONCLUSION

The ermA gene, and gentamicin, ciprofloxacin and tetracycline resistance facilitate biofilm formation in ERY-resistant MRSA isolates and, moreover, ERY-resistant S. aureus isolates with positive biofilm formation exhibited clonality clustering regarding CC239 and CC7.

Prevalence of Genotypes That Determine Resistance of Staphylococci to Macrolides and Lincosamides in Serbia

Macrolides, lincosamides, and streptogramins (MLS) resistance genes are responsible for resistance to these antibiotics in Staphylococcus infections. The purpose of the study was to analyze the distribution of the MLS resistance genes in community- and hospital-acquired Staphylococcus isolates. The MLS resistance phenotypes [constitutive resistance to macrolide–lincosamide–streptogramin B (cMLSb), inducible resistance to macrolide–lincosamide–streptogramin B (iMLSb), resistance to macrolide/macrolide–streptogramin B (M/MSb), and resistance to lincosamide–streptogramin A/streptogramin B (LSa/b)] were determined by double-disc diffusion method. The presence of the MLS resistance genes (ermA, ermB, ermC, msrA/B, lnuA, lnuB, and lsaA) were determined by end-point polymerase chain reaction in 179 isolates of staphylococci collected during 1-year period at the Center for Microbiology of Public Health Institute in Vranje. The most frequent MLS phenotype among staphylococcal isolates, both community-acquired and hospital-acquired, was iMLSb (33.4%). The second most frequent was M/MSb (17.6%) with statistically significantly higher number of hospital-acquired staphylococcal isolates (p < 0.05). MLS resistance was mostly determined by the presence of msrA/B (35.0%) and ermC (20.8%) genes. Examined phenotypes were mostly determined by the presence of one gene, especially by msrA/B (26.3%) and ermC (14.5%), but 15.6% was determined by a combination of two or more genes. M/MSb phenotype was the most frequently encoded by msrA/B (95.6%) gene, LSa/b phenotype by lnuA (56.3%) gene, and iMLSb phenotype by ermC (29.4%) and ermA (25.5%) genes. Although cMLSb phenotype was mostly determined by the presence of ermC (28.9%), combinations of two or more genes have been present too. This pattern was particularly recorded in methicillin-resistant Staphylococcus aureus (MRSA) (58.3%) and methicillin-resistant coagulase-negative staphylococci (MRCNS) (90.9%) isolates with cMLSB phenotype. The msrA/B gene and M/MSb phenotype were statistically significantly higher in hospital-acquired than community-acquired staphylococci strains (p < 0.05). There are no statistically significant differences between staphylococci harboring the rest of MLS resistance genes acquired in community and hospital settings (p > 0.05). The prevalence of iMLSb phenotypes may change over time, so it is necessary to perform periodic survey of MLS resistance phenotypes, particularly where the D-test is not performed routinely.

Diversity of antibiotic resistance genes and staphylococcal cassette chromosome mec elements in faecal isolates of coagulase-negative staphylococci from Nigeria

BACKGROUND

Coagulase-negative staphylococci (CoNS) are opportunistic pathogens found as colonisers of the human gut. This study was carried out to examine the genetic resistance mechanisms in faecal isolates of CoNS. The study investigated 53 non-duplicate CoNS isolates obtained from the fresh stool samples of apparently healthy subjects in the community of Ile-Ife, South-Western Nigeria. Antibiotic susceptibility testing was assessed by the disc diffusion test while antibiotic resistance genes were analysed by PCR. mecA positive isolates were analysed by Staphylococcal Chromosome Cassette mec (SCCmec) and cassette chromosome recombinase (ccr) complex typing methods.

RESULTS

Resistance genes were detected only in isolates that showed resistance by phenotypic screening. The aac(6')-aph(2") gene was detected in all the three isolates resistant to gentamicin. Four of the five erythromycin resistant isolates were positive for the ermC gene, the remaining isolate carried the msrA gene. The tetK gene was detected in 6 of the 7 tetracycline resistant isolates while 4 possessed the tetM gene. Three of the isolates (S. haemolyticus, S. xylosus and S. capitis) had both genes. Several SCCmec types were found: SCCmec I- ccrABβ2-α2 (4 isolates: 3 S. epidermidis, 1 S. warneri), SCCmecIVb- ccrABβ2-α3 (1 isolate: S. epidermidis), SCCmecIVd- ccrABβ2-α3 (8 isolates: 3 S. epidermidis, 2 S. xylosus, 1 S. saprophyticus, 1 S. warneri, 1 S. capitis), and untypable (2 isolates: S. epidermidis).

CONCLUSION

This genetic background could be a reservoir for interspecies gene transfer among CoNS and S. aureus in the intestinal tract.

Implementation of a novel in vitro model of infection of reconstituted human epithelium for expression of virulence genes in methicillin-resistant Staphylococcus aureus strains isolated from catheter-related infections in Mexico

Background

Methicillin-resistant Staphylococcus aureus (MRSA) are clinically relevant pathogens that cause severe catheter-related nosocomial infections driven by several virulence factors.

Methods

We implemented a novel model of infection in vitro of reconstituted human epithelium (RHE) to analyze the expression patterns of virulence genes in 21 MRSA strains isolated from catheter-related infections in Mexican patients undergoing haemodialysis. We also determined the phenotypic and genotypic co-occurrence of antibiotic- and disinfectant-resistance traits in the S. aureus strains, which were also analysed by pulsed-field-gel electrophoresis (PFGE).

Results

In this study, MRSA strains isolated from haemodialysis catheter-related infections expressed virulence markers that mediate adhesion to, and invasion of, RHE. The most frequent pattern of expression (present in 47.6% of the strains) was as follows: fnbA, fnbB, spa, clfA, clfB, cna, bbp, ebps, eap, sdrC, sdrD, sdrE, efb, icaA, and agr. Seventy-one percent of the strains harboured the antibiotic- and disinfectant-resistance genes ermA, ermB, tet(M), tet(K), blaZ, qacA, qacB, and qacC. PFGE of the isolated MRSA revealed three identical strains and two pairs of identical strains. The strains with identical PFGE patterns showed the same phenotypes and genotypes, including the same spa type (t895), suggesting hospital personnel manipulating the haemodialysis equipment could be the source of catheter contamination.

Conclusion

These findings help define the prevalence of MRSA virulence factors in catheter-related infections. Some of the products of the expressed genes that we detected in this work may serve as potential antigens for inclusion in a vaccine for the prevention of MRSA-catheter-related infections.

Detection of macrolide and disinfectant resistance genes in clinical Staphylococcus aureus and coagulase-negative staphylococci

Background

Staphylococcus aureus and Coagulase-negative staphylococci (CoNS) are a major source of infections associated with indwelling medical devices. Many antiseptic agents are used in hygienic handwash to prevent nosocomial infections by Staphylococci. Our aim was to determine the antibiotic susceptibility and resistance to quaternary ammonium compound of 46 S. aureus strains and 71 CoNS.

Methods

S. aureus (n = 46) isolated from auricular infection and CoNS (n = 71), 22 of the strains isolated from dialysis fluids and 49 of the strains isolated from needles cultures were investigated. Erythromycin resistance genes (ermA, ermB, ermC, msrA and mef) were analysed by multiplex PCR and disinfectant-resistant genes (qacA, qacB, and qacC) were studied by PCR-RFLP.

Results

The frequency of erythromycin resistance genes in S. aureus was: ermA+ 7.7%, ermB+ 13.7%, ermC+ 6% and msrA+ 10.2%. In addition, the number of positive isolates in CoNS was respectively ermA+ (9.4%), ermB+ (11.1%), ermC+ (27.4%), and msrA+ (41%). The MIC analyses revealed that 88 isolates (74%) were resistant to quaternary ammonium compound-based disinfectant benzalkonium chloride (BC). 56% of the BC-resistant staphylococcus isolates have at least one of the three resistant disinfectants genes (qacA, qacB and qacC). Nine strains (7.7%) among the CoNS species and two S. aureus strains (2%) harboured the three-qac genes. In addition, the qacC were detected in 41 strains.

Conclusions

Multi-resistant strains towards macrolide and disinfectant were recorded. The investigation of antibiotics and antiseptic-resistant CoNS may provide crucial information on the control of nosocomial infections.

Phenotypic and genotypic study of macrolide, lincosamide and streptogramin B (MLSB) resistance in clinical isolates of Staphylococcus aureus in Tehran, Iran

Background

Resistance to antimicrobial agents among Staphylococcus aureus is an increasing problem. Two common genes responsible for resistance to macrolide, lincosamide and streptogramin B (MLS_B) antibiotics are the ermA and ermC genes. Three resistance phenotypes have been detected to these antibiotics: strains containing cMLS_B (constitutive MLS_B) and iMLS_B (inducible MLS_B), which are resistant to macrolide, lincosamide and streptogramin B antibiotics, and MS, which is only resistant to macrolide and streptogramin B antibiotics. The aim of this study was to determine the prevalence of MLS_B phenotypes and genotypes in erythromycin-resistant strains of S. aureus isolated from patients in 4 university hospitals in Tehran, Iran.

Material/Methods

S. aureus strains were isolated from various clinical specimens and identified by routine phenotypic methods and PCR for nuc gene. Erythromycin resistance was determined by disk diffusion testing. Prevalence of MLS_B phenotypes was determined by use of the D-test. ermA and ermC genes were detected by PCR.

Results

Altogether, 126 erythromycin-resistant strains of S. aureus were detected. Prevalence of cMLS_B, iMLS_B and MS resistance phenotypes were 92.8%, 6.4%, and 0.8%, respectively; 60.3% of strains had ermA gene and 54.8% ermC gene; 61 strains (48.4%) contained 2 studied erm genes and 42 strains (33.3%) did not have any studied erm genes.

Conclusions

Due to the high prevalence of clindamycin resistance among S. aureus isolated from patients in Iran, we recommend clindamycin therapy only after proper antimicrobial susceptibility testing.

Prevalence of resistance phenotypes and genotypes to macrolide, lincosamide and streptogramin antibiotics in Gram-positive cocci isolated in Tunisian Bone Marrow Transplant Center

To investigate the prevalence of resistance to macrolide, lincosamide and streptogramin (MLS) antibiotics in Gram-positive cocci isolated in a Bone Marrow Transplant Center of Tunisia, we tested the antibiotic susceptibility of 172 clinical isolates of Staphylococcus epidermidis, Streptococcus mitis and Enterococcus faecium to macrolide erythromycin and spiramycin, the lincosamide clindamycin and the streptogramin pristinamycin. These three groups of organisms were mostly resistant to macrolides and lincosamide, but were commonly susceptible to pristinamycin. The resistance phenotypes of erythromycin-resistant isolates were determined by the five-disc test with erythromycin, spiramycin, lincomycin, clindamycin and pristinamycin, which showed that most exhibited constitutive MLS resistance. In order to determine the prevalence of the resistance genotypes and the resistance mechanisms, the prevalence of the erythromycin resistance methylase (erm) (A), erm(B), erm(C), msr(A) and macrolide efflux (mef) (A) genes in the erythromycin-resistant isolates was identified by polymerase chain reaction (PCR) analysis. The resistance was due mainly to the presence of ermB in E. faecium (80%), ermC in S. epidermidis (53%) and mefA in S. mitis (65%).

Distribution and expression of macrolide resistance genes in coagulase-negative staphylococci

In total, 494 isolates of coagulase-negative staphylococci (CoNS) were identified to the species level by biochemical tests and sodA sequencing. Erythromycin resistance phenotypes were determined and specific resistance genes were identified by PCR. The prevalence of erythromycin resistance varied widely among staphylococcal species, from 0% in Staphylococcus lugdunensis to almost 90% in Staphylococcus haemolyticus. Most (63%) erythromycin-resistant isolates carried constitutively expressed erm(C) as the sole resistance determinant, with the notable exception of Staphylococcus hominis subsp. hominis, which carried inducible erm(C). The erm(A) and erm(B) determinants were comparatively rare. The msr(A) gene was carried by 20-30% of all erythromycin-resistant isolates, with little variation among species, and was combined in 16.7% of isolates with mph(C), a resistance gene of unknown clinical relevance found previously in isolates of veterinary origin. No erythromycin resistance that could not be attributed to the genes investigated was detected. It was concluded that the presence of methylases cannot be assumed in CoNS isolates that appear erythromycin-resistant and clindamycin-susceptible; thus, methods that detect the export mechanism should be used with clinically significant isolates to indicate whether use of clindamycin may be effective. In Staphylococcus epidermidis and S. haemolyticus, 46% and 66%, respectively, of erythromycin-resistant, clindamycin-susceptible isolates were susceptible to clindamycin therapy.

Prevalence and mechanisms of macrolide resistance among Staphylococcus epidermidis isolates from neutropenic patients in Tunisia

The prevalence of macrolide-lincosamide-streptogramin (MLS) resistance phenotypes was determined among erythromycin-resistant Staphylococcus epidermidis isolates collected at the Bone Marrow Transplant Centre, Tunisia during 2002. The erm(A), erm(B), erm(C), msrA, mefA and icaA genes were detected by PCR. The vga, vgb and vat genes were amplified from pristinamycin-resistant isolates. The icaA gene was detected in 76.5% of 34 isolates examined in detail. The erm(C) (53%) and erm(A) (32%) genes predominated because of clonal dissemination, followed by msrA (15%). Gene distribution was related to the methicillin resistance pattern. The vga gene was present in combination with erm(A) in three isolates.

A guide to antibiotic resistance in bacterial skin infections

The emergence of bacterial resistance to commonly used antibiotics is not new. In this review we have tried to cover the ever increasing problems facing the treatment and containment of bacterial skin infections. We have tried to give an overview of the varied mechanisms by which bacteria gain and spread antimicrobial resistance, whilst dealing with the patterns of resistance exhibited by some of the commonly encountered organisms. Where there is evidence, we have formulated an approach on how to tackle antibiotic resistance. Where there is a lack of evidence we have formulated what we perceive to be appropriate guidelines.

Trends in antimicrobial susceptibility in relation to antimicrobial usage and presence of resistance genes in Staphylococcus hyicus isolated from exudative epidermitis in pigs

From 1996 to 2001 a total of 467 Staphylococcus hyicus isolates from exudative epidermitis (EE) in pigs in Denmark were examined for susceptibility to 13 different antimicrobial agents. The presence of selected genes encoding macrolide (erm(A), erm(B) and erm(C)), penicillin (blaZ), streptogramin (vat, vga, vga(B), vat(B), vat(D) and vat(E)), streptomycin (aadE) and tetracycline resistance (tet(K), tet(L), tet(M) and tet(O)) were determined in selected isolates. The occurrence of erythromycin resistance increased from 33% in 1996 to a maximum of 62% in 1997 and decreased to 26% in 2001. Resistance to sulphametazole increased from 17% in 1996 to 30% in 1998 but has since decreased to 4% in 2001. Resistance to trimethoprim increased to 51% in 1997 and decreased to 21% in 2001. Resistance to tetracycline (21-31%) remained relatively constant during 1996-2000, but increased to 47% in 2001. Resistance to penicillin (54-75%) streptomycin (33-53%) and tetracycline (21-47%) remained relatively constant over the time investigated. All 48 penicillin resistant isolates examined contained the blaZ gene and 40 (85%) of the streptomycin resistant isolates the aadE gene. It was not possible to detect any streptogramin resistance gene in four streptogramin resistant isolates. Of the 55 erythromycin resistant isolates examined, five contained erm(A), 13 erm(B), 35 erm(C) and two both erm(A) and erm(C). The presence of erm(B) was confirmed by hybridization to plasmid profiles in all 13 PCR-positive isolates. Of 52 tetracycline resistant isolates examined, two contained tet(L), 38 tet(K) and 12 both tet(K) and tet(L).

Molecular detection of antimicrobial resistance

The determination of antimicrobial susceptibility of a clinical isolate, especially with increasing resistance, is often crucial for the optimal antimicrobial therapy of infected patients. Nucleic acid-based assays for the detection of resistance may offer advantages over phenotypic assays. Examples are the detection of the methicillin resistance-encoding mecA gene in staphylococci, rifampin resistance in Mycobacterium tuberculosis, and the spread of resistance determinants across the globe. However, molecular assays for the detection of resistance have a number of limitations. New resistance mechanisms may be missed, and in some cases the number of different genes makes generating an assay too costly to compete with phenotypic assays. In addition, proper quality control for molecular assays poses a problem for many laboratories, and this results in questionable results at best. The development of new molecular techniques, e.g., PCR using molecular beacons and DNA chips, expands the possibilities for monitoring resistance. Although molecular techniques for the detection of antimicrobial resistance clearly are winning a place in routine diagnostics, phenotypic assays are still the method of choice for most resistance determinations. In this review, we describe the applications of molecular techniques for the detection of antimicrobial resistance and the current state of the art.

Associations between the use of antimicrobial agents for growth promotion and the occurrence of resistance among Enterococcus faecium from broilers and pigs in Denmark, Finland, and Norway

This study compares the susceptibility of Enterococcus faecium isolated from pigs and poultry in Denmark, Finland, and Norway to antimicrobial agents used for growth promotion. E. faecium was isolated from 211 broilers and 55 pigs in Denmark in 1997, from Norwegian 55 poultry farms (turkey and broiler farms) and 4 swine farms between 1995 and 1997, and Finnish poultry (52) and swine (43) in 1996 and examined for susceptibility to avilamycin, avoparcin, bacitracin, flavomycin, monensin, salinomycin, spiramycin, tylosin, and virginiamycin. Only a limited number of isolates were categorized as resistant to monensin or salinomycin. In general, an association between the usage of antimicrobial agents in the respective countries and the occurrence of associated resistance was observed. Resistance to avilamycin was frequently observed among isolates from broilers in Denmark, where avilamycin has been used, whereas all isolates from Finland and Norway, where these drugs have not been used, were susceptible. The same phenomenon could be observed for avoparcin, bacitracin, tylosin, and virginiamycin; resistance was frequently observed among isolates from where these antimicrobials have been widely used, but rarely among isolates from where the use has been limited. Also for avoparcin and bacitracin, an association between usage and occurrence of resistance was observed. All isolates categorized as avoparcin resistant contained the vanX gene; isolates from broilers had the T variant in position 8,234 and isolates from pigs the G variant. Three (1%) of the 222 isolates resistant to tylosin contained the ermA gene and 196 (88%) ermB. Sixteen (11%) of the 146 virginiamycin-resistant isolates from broilers and two (7%) of the 27 virginiamycin-resistant isolates from pigs in Denmark contained the satA gene, whereas satA was not observed among any of the virginiamycin-resistant isolates from Finland. A total of 72% of the virginiamycin-resistant E. faecium from broilers in Denmark and all nine virginiamycin-resistant E. faecium from Finland contained satG. This gene was also observed among two (7%) of the virginiamycin-resistant isolates from pigs in Denmark. This study indicates that the use of antimicrobial agents for growth promotion in Denmark, Finland, and Norway have selected for resistance to most of these drugs among E. faecium in food animals.

Comparative molecular analysis of erythromycin-resistance determinants in staphylococcal isolates of poultry and human origin

The ermA, ermB, ermC and msrA/msrB genes were detected in multidrug-resistant Staphylococcus spp. strains by PCR. Among 25 human clinical staphylococcal isolates the ermA, ermB, ermC and the msrA/msrB genes were detected in 88, 72, 4 and 100% of the strains, respectively. Among 24 poultry isolates the ermA, ermB, ermC and the msrA/msrB genes were detected in 100, 16.6, 50 and 12.5% of the strains, respectively. The ermA gene was found exclusively on the chromosome, whereas the ermC gene was found on 2.4-4.2 kb plasmids. Restriction fragment length polymorphism (RFLP) analysis of the ermA gene with Eco RI revealed five patterns (25.0, 21.0, 10.5, 6.2 and 4. 8 kb) for the clinical strains and two (8.0 and 6.2 kb) for the poultry strains. The 6.2 kb RFLP pattern, in both the poultry and human clinical isolates, indicates a common lineage for the ermA gene.

Multiplex PCR assays for the detection of clinically relevant antibiotic resistance genes in staphylococci isolated from patients infected after cardiac surgery. The ESPRIT Trial

Multiresistant staphylococci (82 Staphylococcus aureus and 114 coagulase-negative staphylococci) were characterized by testing with rapid multiplex polymerase chain reaction (PCR) assays for species identification and detection of associated antibiotic resistance genes. These 196 staphylococci were isolated from 149 adult patients who developed wound infection after elective coronary artery bypass grafts and/or valve surgery. The multiplex PCR assays allowed identification of the most common staphylococcal species with S. aureus- and Staphylococcus epidermidis-specific primers as well as the detection of the erythromycin resistance genes ermA, ermB, ermC and msrA, the aminoglycoside resistance gene aac(6')-aph(2"), the oxacillin resistance gene mecA and the penicillin resistance gene blaZ. There was a very good correlation between the genotypic analysis by PCR and the phenotype determined by standard methods of susceptibility testing and identification of staphylococcal species: 100% for erythromycin resistance, 98.0% for gentamicin resistance, 99.0% for oxacillin resistance, 100% for penicillin resistance and 100% for S. aureus and S. epidermidis species identification. This study suggests that the incidence and distribution of the tested clinically relevant antibiotic resistance genes in staphylococci associated with infections after cardiac surgery do not differ from those in strains from other infections. These multiplex PCR assays may be used as diagnostic tools to replace or complement standard methods of susceptibility testing and identification of staphylococci.

Characterization of erythromycin-resistant methylase genes from multiple antibiotic resistant Staphylococcus spp isolated from milk samples of lactating cows

OBJECTIVE

To isolate and characterize erythromycin-resistant methylase genes in multiple-antibiotic resistant staphylococci isolated from milk samples.

ANIMALS

300 lactating cows.

PROCEDURE

23 erythromycin-resistant staphylococci were isolated from milk samples of 300 lactating cows. The prevalence of erythromycin-resistant methylase (erm) genes, ermC and ermA genes, and the multicomponent macrolide efflux pump in staphylococci msrA genes were identified and characterized by use of multiplex polymerase chain reaction (PCR), Southern hybridization, restricted fragment length polymorphism (RFLP) analysis, and dot-blot hybridization.

RESULTS

Biochemical characterization indicated that 3 of 23 (13%) isolates were coagulase-positive Staphylococcus aureus, and the rest were coagulase-negative. Multiplex PCR resulted in amplification of a 520-base pair (bp) region of the ermC gene from the cell lysates of a strain of S simulans M-21 and S sciuri M-28. The ermC gene in both isolates was found on a 3-kilobase plasmid. The ermA gene was found on the chromosome of 21 isolates, and 6 RFLP patterns were observed. None of the isolates harbored the msrA gene.

CONCLUSIONS

Erythromycin-resistant Staphylococcus spp isolated from milk samples of lactating cows may serve as reservoirs of erm genes homologous to those described in human isolates. However, the chromosomal insert patterns and prevalence of these genes, the sizes of plasmids harboring the genes, and the number of inserts of the genes (copy number) may differ from that of human isolates.

Antimicrobial susceptibility and presence of resistance genes in staphylococci from poultry

The species distribution, susceptibility to 19 antimicrobial agents and presence of selected genes encoding resistance to macrolides, streptogramins and tetracyclines were examined among 118 staphylococcal isolates from infections of poultry in Denmark. Isolates were identified using a combination of conventional biochemical testing and 16S rDNA sequencing. The most common species were Staphylococcus aureus (83), Staphylococcus hyicus (11), Staphylococcus xylosus (9) and Staphylococcus cohnii (6). The isolates were susceptible to most antimicrobials tested. A high frequency of S. aureus (30%) was resistant to ciprofloxacin. Only six (7%) S. aureus isolates and one Staphylococcus saprophyticus were penicillin resistant. Resistance to sulphamethoxazole was observed among 16 (19%) of S. aureus isolates and two coagulase negative staphylococci (CNS). Twenty (24%) of the S. aureus isolates were resistant to erythromycin and 19 of these isolates contained the ermA gene, whereas the remaining isolate contained the ermC gene. Eleven (48%) of the novobiocin resistant CNS were resistant to erythromycin and all these isolates contained the ermA gene. Two isolates identified as S. xylosus, were found to be resistant to streptogramins and both contained the vatB- and the vgaB-genes. Thirty-nine (47%) of the S. aureus isolates, three of nine S. hyicus and eight of the 23 novobiocin resistant CNS were tetracycline resistant and all contained the tet(K) gene. A single S. aureus isolate also contained the tet(M) gene. The present study showed a frequent occurrence of resistance to fluoroquinolones, tetracycline and macrolides among staphylococci isolated from broilers in Denmark, whereas the occurrence of resistance to other antimicrobial agents remains low. Similar genes, encoding resistance to erythromycin, tetracycline and streptogramins to those previously observed, were detected.

Correlation between the resistance genotype determined by multiplex PCR assays and the antibiotic susceptibility patterns of Staphylococcus aureus and Staphylococcus epidermidis

Clinical isolates of Staphylococcus aureus (a total of 206) and S. epidermidis (a total of 188) from various countries were tested with multiplex PCR assays to detect clinically relevant antibiotic resistance genes associated with staphylococci. The targeted genes are implicated in resistance to oxacillin (mecA), gentamicin ¿aac(6')-aph(2"), and erythromycin (ermA, ermB, ermC, and msrA). We found a nearly perfect correlation between genotypic and phenotypic analysis for most of these 394 strains, showing the following correlations: 98% for oxacillin resistance, 100% for gentamicin resistance, and 98.5% for erythromycin resistance. The discrepant results were (i) eight strains found to be positive by PCR for mecA or ermC but susceptible to the corresponding antibiotic based on disk diffusion and (ii) six strains of S. aureus found to be negative by PCR for mecA or for the four erythromycin resistance genes targeted but resistant to the corresponding antibiotic. In order to demonstrate in vitro that the eight susceptible strains harboring the resistance gene may become resistant, we subcultured the susceptible strains on media with increasing gradients of the antibiotic. We were able to select cells demonstrating a resistant phenotype for all of these eight strains carrying the resistance gene based on disk diffusion and MIC determinations. The four oxacillin-resistant strains negative for mecA were PCR positive for blaZ and had the phenotype of beta-lactamase hyperproducers, which could explain their borderline oxacillin resistance phenotype. The erythromycin resistance for the two strains found to be negative by PCR is probably associated with a novel mechanism. This study reiterates the usefulness of DNA-based assays for the detection of antibiotic resistance genes associated with staphylococcal infections.

Phenotypes of staphylococcal resistance to macrolides, lincosamides and streptogramin B (MLS) in a Turkish university hospital

Resistance to macrolides, lincosamides and streptogramin B (MLS) which is expressed either constitutively or inducibly, is mediated by erm genes (erm A, erm B, and erm C in staphylococci). The transposon TN 554, harbouring the erm A gene also encodes spectinomycin resistance. In Turkey, data related to MLS resistance phenotypes of staphylococci are not available. In this study, we screened 500 consecutive clinical isolates of staphylococci isolated in Hacettepe University Hospital, for MLS and spectinomycin resistance by the standard disk diffusion method. All MLS-resistant isolates were further tested for spectinomycin susceptibility by the agar screening method. Of 500 staphylococcal isolates, 368 (73.6%) were susceptible and 132 (26.4%) were resistant to MLS antibiotics. Ninety-one (18.2%) of the resistant isolates exhibited a constitutive resistance pattern, whereas 40 were inducibly resistant. MS (resistance to macrolides and lincosamides only) resistance was detected in only one isolate (0.2%). Of 40 inducibly resistant isolates, 21 were found to be resistant to spectinomycin by both the disk diffusion and agar screening tests, probably indicating a presence of the erm A gene. These results suggest that MLS resistance has been considerably high among clinical isolates of staphylococci in our hospital. On the whole, constitutive resistance was the pattern most frequently encountered. In contrast, MS resistance was very rare. Further epidemiological and molecular investigations are required for clarification of the data presented.

Simultaneous detection of erythromycin-resistant methylase genes ermA and ermC from Staphylococcus spp. by multiplex-PCR

A comparative analysis of the two most dominant erythromycin-resistance determinant genes in Staphylococcus sppnamely, the ermA and ermC genes, was carried out. Sixty erythromycin-resistant strains of Staphylococcus spp. were tested, of which 24 were avian and 36 were clinical isolates. Our results indicated the prevalence of ermA over the ermC gene as opposed to the widely held opinion of the ermC gene being the most dominant resistance determinant gene. A multiplex-PCR assay was developed to detect the presence of ermA and ermC genes. Two pairs of primers, specific for the detection of ermA and ermC genes, were used in a multiplex-polymerase chain reaction (PCR) assay to yield amplified DNA products of 610 and 520 bp, respectively. Their digestion with restriction enzyme FokI that yielded a 477 bp and a 132 bp digestion product for ermA and a 333 bp and a 187 bp digestion product for ermC confirmed the authenticity of PCR products. The method could be used to amplify the ermA and ermC genes with as little as 5 pg of template DNA. The use of excess primers or the template DNA resulted in gene-specific amplification and no non-specific amplification was observed by changing the primer to primer or template to primer ratios. Furthermore, no amplification from erythromycin-sensitive S. aureus strain was observed. Using this assay, the poultry strains were found to contain either ermA alone (50%) or a combination of ermA (100%) and ermC (50%) both. The clinical strains contained either ermA (94.5%) or ermC (5.5%) but never both. The gene-specific internal probes were also used to verify the above findings and a high degree of correlation between the multiplex PCR and Southern hybridization data was observed.

Biochemical and molecular characterization of erthromycin-resistant avian Staphylococcus spp. isolated from chickens

The epidemiology of the two common erythromycin-resistant methylase (erm) genes ermC and ermA was analyzed in 12 coagulase-negative Staphylococcus spp. and 34 coagulase-positive Staphylococcus spp. isolated from chicken. Southern hybridization indicated that only 2 of the 12 coagulase-negative Staphylococcus spp. strains contained the ermC gene on the plasmid; 1 strain of Staphylococcus xylosus harbored the ermC gene on a 2.5-kb plasmid, and 1 strain of Staphylococcus cohnii harbored the gene on a 4.0-kb plasmid. Twelve of the 34 strains of Staphylococcus aureus contained the ermC gene. Eleven of these strains had the ermC gene on a 2.5-kb plasmid, and 1 strain had the gene on a 4.0-kb plasmid. Ten of the 12 coagulase-negative Staphylococcus spp. and 22 of the 34 coagulase-positive Staphylococcus spp. harbored the ermA gene exclusively on the chromosome. Two different ermA EcoRI restriction fragment length polymorphisms (RFLP) were identified. A majority of the isolates was found to have two chromosomal inserts (8.0- and 6.2-kb EcoRI fragments) of ermA. One strain of S. aureus had different chromosomal inserts (6.4- and 5.8-kb EcoRI fragments) of ermA. Our results indicate that either the ermC or ermA gene, homologous to those described in human isolates, was present in all avian Staphylococcus spp. and that ermA was the predominant gene in coagulase-negative and coagulase-positive avian Staphylococcus spp. The size and copy numbers of the ermA gene were different from its human counterpart.

Characterization of erythromycin-resistant isolates of Staphylococcus aureus recovered in the United States from 1958 through 1969

We tested 16 erythromycin-resistant clinical isolates of S. aureus, recovered from patients hospitalized in the United States from 1958 to 1969, for the presence of ermA, ermB, and ermC by using PCR. Fifteen of 16 isolates contained at least one copy of ermA; the remaining isolate, which was also clindamycin resistant, contained ermB. Eight of the 15 isolates harboring ermA, all of which were inducible, contained a single copy of the gene in the chromosome, while the remaining seven isolates had two copies of the gene. ermB was plasmid encoded and mediated constitutive resistance to erythromycin.

Antibiotic resistance genes in coagulase-negative staphylococci isolated from food

Coagulase-negative staphylococci were isolated from different raw milk cheeses and raw meat products and screened for their antibiotic resistances. They were identified as Staphylococcus xylosus, S. lentus, S. caprae, S. epidemidis and S. haemolyticus. The most frequent resistances found were those to chloramphenicol, tetracycline, erythromycin and lincomycin. They have been characterized on the molecular level. The chloramphenicol resistance genes were localized in several S. xylosus and S. caprae on plasmids with sizes ranging from 3.8-kb to 4.3-kb and were identified as chloramphenicol acetyltransferase (cat). All the tetracycline resistant strains were identified as S. xylosus and harboured a 4.4-kb plasmid carrying the tetracycline efflux resistance gene (tetK). The two erythromycin/lincomycin resistant S. caprae and S. epidermidis strains did not hybridize with the MLSB resistance genes ermAM, ermA, ermB and ermC. Three erythromycin resistant Staphylococcus sp. strains harboured an erythromycin efflux resistance gene (msr) localized twice on a 18-kb plasmid and once on the chromosome. A S. haemolyticus strain showing resistance to both lincomycin and clindamycin harboured a linA gene-carrying 2.2-kb plasmid. Further resistances to gentamicin, penicillin and kanamycin were less frequently observed and yet not characterized on a molecular level.