Analysis of gyrA and parC mutations in enterococci from environmental samples with reduced susceptibility to ciprofloxacin

Non-faecium non-faecalis enterococci: a review of clinical manifestations, virulence factors, and antimicrobial resistance

SUMMARYEnterococci are a diverse group of Gram-positive bacteria that are typically found as commensals in humans, animals, and the environment. Occasionally, they may cause clinically relevant diseases such as endocarditis, septicemia, urinary tract infections, and wound infections. The majority of clinical infections in humans are caused by two species: Enterococcus faecium and Enterococcus faecalis. However, there is an increasing number of clinical infections caused by non-faecium non-faecalis (NFF) enterococci. Although NFF enterococcal species are often overlooked, studies have shown that they may harbor antimicrobial resistance (AMR) genes and virulence factors that are found in E. faecium and E. faecalis. In this review, we present an overview of the NFF enterococci with a particular focus on human clinical manifestations, epidemiology, virulence genes, and AMR genes.

Vancomycin Resistance in Enterococcus and Staphylococcus aureus

Enterococcus faecalis, Enterococcus faecium and Staphylococcus aureus are both common commensals and major opportunistic human pathogens. In recent decades, these bacteria have acquired broad resistance to several major classes of antibiotics, including commonly employed glycopeptides. Exemplified by resistance to vancomycin, glycopeptide resistance is mediated through intrinsic gene mutations, and/or transferrable van resistance gene cassette-carrying mobile genetic elements. Here, this review will discuss the epidemiology of vancomycin-resistant Enterococcus and S. aureus in healthcare, community, and agricultural settings, explore vancomycin resistance in the context of van and non-van mediated resistance development and provide insights into alternative therapeutic approaches aimed at treating drug-resistant Enterococcus and S. aureus infections.

Phenotypic and genotypic antimicrobial resistance profiles of fecal lactobacilli from domesticated pigeons in Poland

Lactobacillus species play an important role in the host and although they are non-pathogenic, they could act as reservoirs for antibiotic resistance genes, with the potential risk of transfer to other bacteria inhabiting the gastrointestinal tract. The aim of this study was to identify Lactobacillus species derived from feces of domesticated pigeons and to characterize their phenotypic and genotypic antimicrobial resistance (AMR) profiles. A total of 57 Lactobacillus isolates were classified into six species using the MALDI-TOF technique and 16S rDNA restriction analysis. Strains of L. ingluviei (31%), L. salivarius (28%) and L. agilis (23%) were the dominant species isolated. Determination of antimicrobial susceptibility by the microdilution broth method showed widespread resistance to kanamycin (89%), tetracycline (84%), streptomycin (63%), and enrofloxacin (37%). Less than 30% of the isolates were resistant to erythromycin, lincosamides, gentamycin, chloramphenicol and vancomycin. Over half (51%) of the lactobacilli were classified as multidrug resistant. Tet genes were detected in 79% of isolates; the lnuA, cat, ermB, ermC, ant(6)-Ia, ant(4')-Ia, and int-Tn genes were found at a lower frequency. Sequence analysis of the quinolone resistance-determining region (QRDR)of the gyrA gene showed that fluoroquinolone resistance in lactobacilli was the result of a mutation that lead to a change in the amino acid sequence (Ser83→Tyr/Leu/Phe). Domesticated pigeons could be a reservoir for AMR Lactobacillus strains and AMR genes.

Antimicrobial resistance monitoring of commensal Enterococcus faecalis in broiler breeders

Enterococcus faecalis (E. faecalis) has rapidly acquired resistance to multiple antimicrobials, and the antimicrobial resistance of E. faecalis from broiler breeders has been implicated in its vertical transmission to their offspring. The objective of this study was to investigate the antimicrobial resistance and genetic diversity of commensal E. faecalis isolated from the broiler breeder farms. Among a total of 229 E. faecalis isolates from 9 broiler breeder farms, the highest resistance rate was observed in tetracycline (78.2%), followed by doxycycline (58.1%) and erythromycin (43.7%), and the prevalence of antimicrobial resistance showed significant differences among the 9 broiler breeder farms (P < 0.05). The tetM gene (77.1%) and ermB gene (85.0%) were detected at the highest levels in 179 TE-and 100 E-resistant isolates, respectively. Twenty-four high-level gentamicin-resistant isolates carried aac(6″)Ie-aph(2″)-la gene, and 9 high-level ciprofloxacin-resistant isolates showed point mutations in both gyrA and parC genes. All high-level gentamicin-resistant or high-level ciprofloxacin-resistant isolates showed one of the two different virulence gene patterns, ace-asa1-efaA-gelE complex or ace-efaA-gelE complex. These results indicate that constant epidemiological monitoring at the breeder level is required to prevent the pyramidal transmission of antimicrobial-resistant E. faecalis.

Application of biofertilizers increases fluoroquinolone resistance in Vibrio parahaemolyticus isolated from aquaculture environments

Antimicrobial resistance genes in aquaculture environments have attracted wide interest, since these genes pose a severe threat to human health. This study aimed to explore the possible mechanisms of the ciprofloxacin resistance of Vibrio parahaemolyticus (V. parahaemolytiucs) in aquaculture environments, which may have been affected by the biofertilizer utilization in China. Plasmid-mediate quinolone resistance (PMQR) genes, representative (fluoro)quinolones (FNQs), and ciprofloxacin-resistance isolates in biofertilizer samples were analyzed. The significantly higher abundance of oqxB was alarming. The transferable experiments and Southern blot analysis indicated that oqxB could spread horizontally from biofertilizers to V. parahaemolyticus, and two (16.7%) trans-conjugants harboring oqxB were provided by 12 isolates that successfully produced OqxB. To the best of our knowledge, this study is the first to report PMQR genes dissipation from biofertilizers to V. parahaemolyticus in aquaculture environments. The surveillance, monitoring and control of PMQR genes in biofertilizers are warranted for seafood safety and human health.

Antibiotic Resistance of LACTOBACILLUS Strains

The study provides phenotypic and molecular analyses of the antibiotic resistance in 20 Lactobacillus strains including 11 strains newly isolated from fermented plant material. According to the results of disc diffusion method, 90% of tested lactobacilli demonstrated sensitivity to clindamycin and 95% of strains were susceptible to tetracycline, erythromycin, and rifampicin. Ampicillin and chloramphenicol were found to inhibit all bacteria used in this study. The vast majority of tested strains revealed phenotypic resistance to vancomycin, ciprofloxacin, and aminoglycosides. Most of Lactobacillus strains showed high minimum inhibitory concentrations (MICs) of cefotaxime, ceftriaxone, and cefazolin and therefore were considered resistant to cephalosporins. All the strains exhibited multidrug resistance. The occurrence of resistance genes was associated with phenotypic resistance, with the exception of phenotypically susceptible strains that contained genes for tetracycline (tetK, tetL) and erythromycin (ermB, mefA) resistance. The vanX gene for vancomycin resistance was among the most frequently identified among the lactobacilli (75% of strains), but the occurrence of the parC gene for ciprofloxacin resistance was sporadic (20% of strains). Our results mainly evidence the intrinsic nature of the resistance to aminoglycosides in lactobacilli, though genes for enzymatic modification of streptomycin aadA and aadE were found in 20% of tested strains. The occurrence of extended spectrum beta-lactamases (ESBL) was unknown in Lactobacillus, but our results revealed the blaTEM gene in 80% of strains, whereas blaSHV and blaOXA-1 genes were less frequent (20% and 15% of strains, respectively).

Long-Term Colonization Dynamics of Enterococcus faecalis in Implanted Devices in Research Macaques

Enterococcus faecalis is a common opportunistic pathogen that colonizes cephalic recording chambers (CRCs) of macaques used in cognitive neuroscience research. We previously characterized 15 E. faecalis strains isolated from macaques at the Massachusetts Institute of Technology (MIT) in 2011. The goal of this study was to examine how a 2014 protocol change prohibiting the use of antimicrobials within CRCs affected colonizing E. faecalis strains. We collected 20 E. faecalis isolates from 10 macaques between 2013 and 2017 for comparison to 4 isolates previously characterized in 2011 with respect to the sequence type (ST) distribution, antimicrobial resistance, biofilm formation, and changes in genes that might confer a survival advantage. ST4 and ST55 were predominant among the isolates characterized in 2011, whereas the less antimicrobial-resistant lineage ST48 emerged to dominance after 2013. Two macaques remained colonized by ST4 and ST55 strains for 5 and 4 years, respectively. While the antimicrobial resistance and virulence factors identified in these ST4 and ST55 strains remained relatively stable, we detected an increase in biofilm formation ability over time in both isolates. We also found that ST48 strains were typically robust biofilm formers, which could explain why this ST increased in prevalence. Finally, we identified mutations in the DNA mismatch repair genes mutS and mutL in separate ST55 and ST4 strains and confirmed that strains bearing these mutations displayed a hypermutator phenotype. The presence of a hypermutator phenotype may complicate future antimicrobial treatment for clinically relevant E. faecalis infections in macaques.IMPORTANCEEnterococcus faecalis is a common cause of health care-associated infections in humans, largely due to its ability to persist in the hospital environment, colonize patients, acquire antimicrobial resistance, and form biofilms. Understanding how enterococci evolve in health care settings provides insight into factors affecting enterococcal survival and persistence. Macaques used in neuroscience research have long-term cranial implants that, despite best practices, often become colonized by E. faecalis This provides a unique opportunity to noninvasively examine the evolution of enterococci on a long-term indwelling device. We collected E. faecalis strains from cephalic implants over a 7-year period and characterized the sequence type, antimicrobial resistance, virulence factors, biofilm production, and hypermutator phenotypes. Improved antimicrobial stewardship allowed a less-antimicrobial-resistant E. faecalis strain to predominate at the implant interface, potentially improving antimicrobial treatment outcomes if future clinical infections occur. Biofilm formation appears to play an important role in the persistence of the E. faecalis strains associated with these implants.

Ecological impact of ciprofloxacin on commensal enterococci in healthy volunteers

Background

The ecological impact of ciprofloxacin on commensal enterococci is unknown.

Methods

Forty-eight healthy volunteers received ciprofloxacin from day (D) 0 to D14; stools were collected on D7, D14 and D42. Fluoroquinolone-susceptible and -resistant enterococci (FQ-SE and FQ-RE) were detected and quantified by culture, and identified by MALDI-TOF MS. The relative abundance of FQ-RE over FQ-SE was determined. The genetic basis of fluoroquinolone resistance was deciphered by partial sequencing of gyrA and parC genes. Clonal relatedness was determined by random amplification of polymorphic DNA PCR. Clinical trial no.: NCT00190151.

Results

Enterococci were carried by 47/48 (98%) subjects. Total counts were reduced during ciprofloxacin therapy (4.0 and 3.9 log cfu/g on D7 and D14 versus 5.9 log cfu/g before and 6.9 log cfu/g after treatment; P  < 0.05). Twenty-one out of 48 (44%) carried FQ-RE; among them, 21/21 carried Enterococcus faecium , 19 carried Enterococcus faecalis and 11 carried other species. Five out of 48 (10%) harboured FQ-RE (ciprofloxacin MIC >4 mg/L) before treatment (all E. faecium ), 6 on D7 (3 E. faecium and 3 E. faecalis ), 8 on D14 (4 E. faecium and 4 E. faecalis ) and 10 (21%) on D42 (9 E. faecium and 1 E. faecalis ). The relative abundance of FQ-RE increased from 44% on D0 to 73% and 75% on D7 and D14, respectively. No acquisition of fluoroquinolone resistance among endogenous D0 strains was evidenced. All (14/14) distinct Fluoroquinolone-resistant E. faecalis clones were gyrA / parC double mutants with high-level resistance (ciprofloxacin MIC >64 mg/L). In contrast, 34/35 E. faecium exhibited low-level resistance (ciprofloxacin MIC 4-32 mg/L) with no gyrA / parC mutation, but overexpressed the chromosomal Efm qnr gene. As compared with Fluoroquinolone-susceptible strains, Fluoroquinolone-resistant E. faecium were more frequently ampicillin resistant and Fluoroquinolone-resistant E. faecalis were more highly resistant to gentamicin.

Conclusions

Although intrinsically poorly susceptible to fluoroquinolones, gut populations of enterococci are highly impacted both quantitatively and qualitatively by ciprofloxacin.

Detection of Antibiotic Resistance and Resistance Genes in Enterococci Isolated from Sucuk, a Traditional Turkish Dry-Fermented Sausage

The aim of this study was to isolate enterococci in Sucuk, a traditional Turkish dry-fermented sausage and to analyze isolates for their biodiversity, antibiotic resistance patterns and the presence of some antibiotic resistance genes. A total of 60 enterococci strains were isolated from 20 sucuk samples manufactured without using a starter culture and they were identified as E. faecium (73.3%), E. faecalis (11.7%), E. hirae (8.3%), E. durans (3.3%), E. mundtii (1.7%) and E. thailandicus (1.7%). Most of the strains were found resistant to rifampin (51.67%) followed by ciprofloxacin (38.33%), nitrofurantoin (33.33%) and erythromycin (21.67%). All strains were found susceptible to ampicillin. Only E. faecium FYE4 and FYE60 strains displayed susceptibility to all antibiotics. Other strains showed different resistance patterns to antibiotics. E. faecalis was found more resistant to antibiotics than other species. Most of the strains (61.7%) displayed resistance from between two and eight antibiotics. The ermB, ermC, gyrA, tetM, tetL and vanA genes were detected in some strains. A lack of correlation between genotypic and phenotypic analysis for some strains was detected. The results of this study indicated that Sucuk manufactured without using a starter culture is a reservoir of multiple antibiotic resistant enterococci. Consequently, Sucuk is a potential reservoir for the transmission of antibiotic resistance genes from animals to humans.

Enterococcus casseliflavus septicaemia associated with hepatobiliary infection in a 75-year-old man

Enterococcus casseliflavus is a rare non-faecium, non-faecalis, vancomycin-resistant enterococcus (VRE) that is responsible for up to 2% of all enterococcal infections. Septicaemia with this bacterium is usually seen in older patients with multiple comorbidities who have had several previous hospitalisations. Septicaemia with E. casseliflavus portends a poorer prognosis, and treatment usually involves administration of antienterococcal beta-lactams or anti-VRE medications such as linezolid or daptomycin. We present such a case of a 75-year-old man who developed E. casseliflavus septicaemia of presumably hepatobiliary origin and responded well to treatment with intravenous beta-lactams.

Characterization of Multi-Drug Resistant Enterococcus faecalis Isolated from Cephalic Recording Chambers in Research Macaques (Macaca spp.)

Nonhuman primates are commonly used for cognitive neuroscience research and often surgically implanted with cephalic recording chambers for electrophysiological recording. Aerobic bacterial cultures from 25 macaques identified 72 bacterial isolates, including 15 Enterococcus faecalis isolates. The E. faecalis isolates displayed multi-drug resistant phenotypes, with resistance to ciprofloxacin, enrofloxacin, trimethoprim-sulfamethoxazole, tetracycline, chloramphenicol, bacitracin, and erythromycin, as well as high-level aminoglycoside resistance. Multi-locus sequence typing showed that most belonged to two E. faecalis sequence types (ST): ST 4 and ST 55. The genomes of three representative isolates were sequenced to identify genes encoding antimicrobial resistances and other traits. Antimicrobial resistance genes identified included aac(6’)-aph(2”), aph(3’)-III, str, ant(6)-Ia, tetM, tetS, tetL, ermB, bcrABR, cat, and dfrG, and polymorphisms in parC (S80I) and gyrA (S83I) were observed. These isolates also harbored virulence factors including the cytolysin toxin genes in ST 4 isolates, as well as multiple biofilm-associated genes (esp, agg, ace, SrtA, gelE, ebpABC), hyaluronidases (hylA, hylB), and other survival genes (ElrA, tpx). Crystal violet biofilm assays confirmed that ST 4 isolates produced more biofilm than ST 55 isolates. The abundance of antimicrobial resistance and virulence factor genes in the ST 4 isolates likely relates to the loss of CRISPR-cas. This macaque colony represents a unique model for studying E. faecalis infection associated with indwelling devices, and provides an opportunity to understand the basis of persistence of this pathogen in a healthcare setting.

Antibiotic resistance of Lactobacillus pentosus and Leuconostoc pseudomesenteroides isolated from naturally-fermented Aloreña table olives throughout fermentation process

Antimicrobial resistance of Lactobacillus pentosus (n=59) and Leuconostoc pseudomesenteroides (n=13) isolated from Aloreña green table olives (which are naturally-fermented olives from Málaga, Spain) to 15 antibiotics was evaluated. Most Lb. pentosus (95%) and all Lc. pseudomesenteroides were resistant to at least three antibiotics. Principal component analysis determined that the prevalence of antibiotic resistance in LAB throughout the fermentation process was highly dependent on the fermenter where the fermentation took place. All Lb. pentosus and Lc. pseudomesenteroides strains were highly sensitive to amoxicillin and ampicillin (MIC≤2 μg/ml), and also to chloramphenicol (MIC≤4 μg/ml), gentamicin and erythromycin (MIC≤16 μg/ml). However, they were phenotypically resistant to streptomycin (83-100%, MIC>256 μg/ml), vancomycin and teicoplanin (70-100%, MIC>128 μg/ml), trimethoprim (76% of Lb. pentosus and 15% of Lc. pseudomesenteroides, MIC>128 μg/ml), trimethoprim/sulfomethoxazol (71-100%, MIC>4-64 μg/ml) and cefuroxime (44% of Lb. pentosus and 85% of Lc. pseudomesenteroides, MIC>32-128 μg/ml). Lb. pentosus was susceptible to tetracycline and clindamycin, while 46% of Lc. pseudomesenteroides strains were resistant to these antibiotics. Only Lb. pentosus strains were resistant to ciprofloxacin (70%, MIC>4-64 μg/ml), although no mutations in the quinolone resistance determining regions of the genes encoding GyrA and ParC were found, thus indicating an intrinsic resistance. Similarly, no genes encoding possible transferable resistance determinants for the observed phenotypic resistance were detected by PCR. In some cases, a bimodal distribution of MICs was observed for some antibiotics to which both LAB species exhibited resistance. Nevertheless, such resistances resulted from an intrinsic mechanism, non-transferable or non-acquired resistance determinants which may in part be due to chromosomally encoded efflux pumps (NorA, MepA and MdeA). Results of the present study demonstrate that all Lb. pentosus and Lc. pseudomesenteroides strains lack transferable resistance-related genes (cat, bla, blaZ, ermA, ermB, ermC, msrA/B, ereA, ereB, mphA, mefA, tet(M), tet(O), tet(S), tet(W), tet(L), tet(K), aad(E), aac(6')-Ie-aph(2')-Ia, aph(2')-Ib, aph(2')-Ic, aph(2')-Id, aph(3')-IIIa, ant(4')-Ia, dfrA, dfrD, vanA, vanB, vanC and vanE) and should therefore, according to Qualified Presumption of Safety criteria, be considered safe for future application as starter cultures or as probiotics.

Cornmeal-induced resistance to ciprofloxacin and erythromycin in enterococci

OBJECTIVE

To establish a model ecosystem to study the impact of cornmeal on the appearance and persistence of the erythromycin (ERY)- and ciprofloxacin (CIP)-resistant phenotypes in waterborne enterococci.

METHODS

After the model ecosystem was established, the system was divided into six dose groups, with the addition of 8, 4, 1, 0.25, 0.05, and 0 g L(-1) sterilized cornmeal. System mud samples were collected at 0, 1, 3, 7, 14, 30, 40, 61, and 130 d, and enterococci present in the mud samples were evaluated for their sensitivities to CIP and ERY. PCR was employed to detect genes such as gyrA and ermB. The gyrA gene was sequenced, and codons 83 and 87 were analyzed for mutations.

RESULTS

(1) The addition of 0.05-8 g L(-1) cornmeal had an impact on CIP resistance. The higher the dose of cornmeal added, the larger the impact it generated. Furthermore, the earlier the emergence of CIP-resistant strains, the greater the incidence of drug resistance. The impact of cornmeal on resistance to ERY was less consistent, and the degree of the impact was not in proportion to the dose of cornmeal added. (2) There were no mutations at codons 83 and 87 in the gyrA genes from 102 strains isolated from the model ecosystem. The incidence of ermB-positive strains of ERY-resistant enterococci (28 strains) was 78.6%, and the incidence of ermB-positive strains of ERY-sensitive enterococci (16 strains) was 0%.

CONCLUSIONS

(1) Adding different doses of cornmeal can facilitate resistance to CIP and ERY in waterborne enterococci. In this study, the degree of resistance was related to the cornmeal dose. (2) In the model ecosystem, enterococcal CIP resistance was not caused by a gyrA gene mutation; however, in the vast majority of cases, resistance to ERY was related to the ermB resistance gene.

Assessment of antibiotic resistance in probiotic strain Lactobacillus brevis KB290

Our purpose was to investigate the safety of the probiotic strain Lactobacillus brevis KB290. The European Qualified Presumption of Safety (QPS) evaluation approach was applied to the strain. We determined the strain's antibiotic resistance, verified it at the genetic level, and determined whether it could be transferred to intestinal microflora. Of 14 antibiotics tested, 11 showed MICs within the limits of the QPS criteria. However, the L. brevis KB290 MICs of ciprofloxacin (a fluoroquinolone), tetracycline, and vancomycin were two, four, and eight times, respectively, the breakpoint MICs suggested by the European Scientific Committee on Animal Nutrition, and the MIC of tetracycline was eight times the breakpoint MIC suggested by the European Scientific Panel on Additives and Products or Substances Used in Animal Feed. Using analysis of gapped-genome sequences, we found no known transferable determinants for tetracycline or vancomycin resistance, and we found no mutations in the quinolone resistance-determining regions of the genes encoding GyrA or ParC for ciprofloxacin resistance associated with insertion sequences, integrons, or transposons. These data were confirmed by using PCR primers specific for the respective genes. We assessed the transferability of the resistance traits in conjugation experiments with enterococci and obtained no transconjugants, strongly suggesting that the resistance traits were not transferable. This study demonstrated that the antibiotic resistance observed in L. brevis KB290 was due not to dedicated mechanisms but to intrinsic resistance. According to the QPS criteria, these results provide safety assurance for the ongoing use of L. brevis KB290 as a probiotic.

Antibiotic resistances of starter and probiotic strains of lactic acid bacteria

The antibiotic resistances of 45 lactic acid bacteria strains belonging to the genera Lactobacillus, Streptococcus, Lactococcus, Pediococcus, and Leuconostoc were investigated. The objective was to determine antibiotic resistances and to verify these at the genetic level, as is currently suggested by the European "qualified presumption of safety" safety evaluation system for industrial starter strains. In addition, we sought to pinpoint possible problems in resistance determinations. Primers were used to PCR amplify genes involved in beta-lactam antibiotic, chloramphenicol, tetracycline, and erythromycin resistance. The presence of ribosomal protection protein genes and the ermB gene was also determined by using a gene probe. Generally, the incidences of erythromycin, chloramphenicol, tetracycline, or beta-lactam resistances in this study were low (<7%). In contrast, aminoglycoside (gentamicin and streptomycin) and ciprofloxacin resistances were higher than 70%, indicating that these may constitute intrinsic resistances. The genetic basis for ciprofloxacin resistance could not be verified, since no mutations typical of quinolone resistances were detected in the quinolone determining regions of the parC and gyrA genes. Some starter strains showed low-level ampicillin, penicillin, chloramphenicol, and tetracycline resistances, but no known resistance genes could be detected. Although some strains possessed the cat gene, none of these were phenotypically resistant to chloramphenicol. Using reverse transcription-PCR, these cat genes were shown to be silent under both inducing and noninducing conditions. Only Lactobacillus salivarius BFE 7441 possessed an ermB gene, which was encoded on the chromosome and which could not be transferred in filter-mating experiments. This study clearly demonstrates problems encountered with resistance testing, in that the breakpoint values are often inadequately identified, resistance genes may be present but silent, and the genetic basis and associated resistance mechanisms toward some antibiotics are still unknown.

Biological counterstrike: antibiotic resistance mechanisms of Gram-positive cocci

The development of antibiotic resistance by bacteria is an evolutionary inevitability, a convincing demonstration of their ability to adapt to adverse environmental conditions. Since the emergence of penicillinase-producing Staphylococcus aureus in the 1940s, staphylococci, enterococci and streptococci have proved themselves adept at developing or acquiring mechanisms that confer resistance to all clinically available antibacterial classes. The increasing problems of methicillin-resistant S. aureus and coagulase-negative staphylococci (MRSA and MRCoNS), glycopeptide-resistant enterococci and penicillin-resistant pneumococci in the 1980s, and recognition of glycopeptide-intermediate S. aureus in the 1990s and, most recently, of fully vancomycin-resistant isolates of S. aureus have emphasised our need for new anti-Gram-positive agents. Antibiotic resistance is one of the major public health concerns for the beginning of the 21st century. The pharmaceutical industry has responded with the development of oxazolidinones, lipopeptides, injectable streptogramins, ketolides, glycylcyclines, second-generation glycopeptides and novel fluoroquinolones. However, clinical use of these novel agents will cause new selective pressures and will continue to drive the development of resistance. This review describes the various antibiotic resistance mechanisms identified in isolates of staphylococci, enterococci and streptococci, including mechanisms of resistance to recently introduced anti-Gram-positive agents.