pBMSa1, a plasmid from a dairy cow isolate of Staphylococcus aureus, encodes a lincomycin resistance determinant and replicates by the rolling-circle mechanism

Molecular Mechanisms of Drug Resistance in Staphylococcus aureus

This paper discusses the mechanisms of S. aureus drug resistance including: (1) introduction. (2) resistance to beta-lactam antibiotics, with particular emphasis on the mec genes found in the Staphylococcaceae family, the structure and occurrence of SCCmec cassettes, as well as differences in the presence of some virulence genes and its expression in major epidemiological types and clones of HA-MRSA, CA-MRSA, and LA-MRSA strains. Other mechanisms of resistance to beta-lactam antibiotics will also be discussed, such as mutations in the gdpP gene, BORSA or MODSA phenotypes, as well as resistance to ceftobiprole and ceftaroline. (3) Resistance to glycopeptides (VRSA, VISA, hVISA strains, vancomycin tolerance). (4) Resistance to oxazolidinones (mutational and enzymatic resistance to linezolid). (5) Resistance to MLS-B (macrolides, lincosamides, ketolides, and streptogramin B). (6) Aminoglycosides and spectinomicin, including resistance genes, their regulation and localization (plasmids, transposons, class I integrons, SCCmec), and types and spectrum of enzymes that inactivate aminoglycosides. (7). Fluoroquinolones (8) Tetracyclines, including the mechanisms of active protection of the drug target site and active efflux of the drug from the bacterial cell. (9) Mupirocin. (10) Fusidic acid. (11) Daptomycin. (12) Resistance to other antibiotics and chemioterapeutics (e.g., streptogramins A, quinupristin/dalfopristin, chloramphenicol, rifampicin, fosfomycin, trimethoprim) (13) Molecular epidemiology of MRSA.

Relationship between virulence factors and antimicrobial resistance in Staphylococcus aureus from bovine mastitis

OBJECTIVES

This review summarizes the literature on the role of virulence and antimicrobial resistance genes of Staphylococcus aureus in bovine mastitis, focusing on the association between these characteristics and their implications for public and animal health.

CONCLUSIONS

There is the possibility of antimicrobial resistance gene exchange among different bacteria, which is of serious concern in livestock husbandry, as well as in the treatment of human staphylococcal infections.

Small Antimicrobial Resistance Plasmids in Livestock-Associated Methicillin-Resistant Staphylococcus aureus CC398

Livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) isolates of the clonal complex 398 are often resistant to a number of antimicrobial agents. Studies on the genetic basis of antimicrobial resistance in these bacteria identified SCCmec cassettes, various transposons and plasmids of different sizes that harbor antimicrobial resistance genes. While large plasmids that carry multiple antimicrobial resistance genes – occasionally together with heavy metal resistance genes and/or virulence genes – are frequently seen in LA-MRSA ST398, certain resistance genes are also associated with small plasmids of up to 15 kb in size. These small resistance plasmids usually carry only one, but in rare cases also two or three antimicrobial resistance genes. In the current review, we focus on small plasmids that carry the macrolide-lincosamide-streptogramin B resistance genes erm(C) or erm(T), the lincosamide resistance gene lnu(A), the pleuromutilin-lincosamide-streptogramin A resistance genes vga(A) or vga(C), the spectinomycin resistance gene spd, the apramycin resistance gene apmA, or the trimethoprim resistance gene dfrK. The detailed analysis of the structure of these plasmids allows comparisons with similar plasmids found in other staphylococci and underlines in many cases an exchange of such plasmids between LA-MRSA ST398 and other staphylococci including also coagulase-negative staphylococci.

Antimicrobial Resistance among Staphylococci of Animal Origin

Antimicrobial resistance among staphylococci of animal origin is based on a wide variety of resistance genes. These genes mediate resistance to many classes of antimicrobial agents approved for use in animals, such as penicillins, cephalosporins, tetracyclines, macrolides, lincosamides, phenicols, aminoglycosides, aminocyclitols, pleuromutilins, and diaminopyrimidines. In addition, numerous mutations have been identified that confer resistance to specific antimicrobial agents, such as ansamycins and fluoroquinolones. The gene products of some of these resistance genes confer resistance to only specific members of a class of antimicrobial agents, whereas others confer resistance to the entire class or even to members of different classes of antimicrobial agents, including agents approved solely for human use. The resistance genes code for all three major resistance mechanisms: enzymatic inactivation, active efflux, and protection/modification/replacement of the cellular target sites of the antimicrobial agents. Mobile genetic elements, in particular plasmids and transposons, play a major role as carriers of antimicrobial resistance genes in animal staphylococci. They facilitate not only the exchange of resistance genes among members of the same and/or different staphylococcal species, but also between staphylococci and other Gram-positive bacteria. The observation that plasmids of staphylococci often harbor more than one resistance gene points toward coselection and persistence of resistance genes even without direct selective pressure by a specific antimicrobial agent. This chapter provides an overview of the resistance genes and resistance-mediating mutations known to occur in staphylococci of animal origin.

Lincosamides, Streptogramins, Phenicols, and Pleuromutilins: Mode of Action and Mechanisms of Resistance

Lincosamides, streptogramins, phenicols, and pleuromutilins (LSPPs) represent four structurally different classes of antimicrobial agents that inhibit bacterial protein synthesis by binding to particular sites on the 50S ribosomal subunit of the ribosomes. Members of all four classes are used for different purposes in human and veterinary medicine in various countries worldwide. Bacteria have developed ways and means to escape the inhibitory effects of LSPP antimicrobial agents by enzymatic inactivation, active export, or modification of the target sites of the agents. This review provides a comprehensive overview of the mode of action of LSPP antimicrobial agents as well as of the mutations and resistance genes known to confer resistance to these agents in various bacteria of human and animal origin.

Plasmid-Mediated Antimicrobial Resistance in Staphylococci and Other Firmicutes

In staphylococci and other Firmicutes, resistance to numerous classes of antimicrobial agents, which are commonly used in human and veterinary medicine, is mediated by genes that are associated with mobile genetic elements. The gene products of some of these antimicrobial resistance genes confer resistance to only specific members of a certain class of antimicrobial agents, whereas others confer resistance to the entire class or even to members of different classes of antimicrobial agents. The resistance mechanisms specified by the resistance genes fall into any of three major categories: active efflux, enzymatic inactivation, and modification/replacement/protection of the target sites of the antimicrobial agents. Among the mobile genetic elements that carry such resistance genes, plasmids play an important role as carriers of primarily plasmid-borne resistance genes, but also as vectors for nonconjugative and conjugative transposons that harbor resistance genes. Plasmids can be exchanged by horizontal gene transfer between members of the same species but also between bacteria belonging to different species and genera. Plasmids are highly flexible elements, and various mechanisms exist by which plasmids can recombine, form cointegrates, or become integrated in part or in toto into the chromosomal DNA or into other plasmids. As such, plasmids play a key role in the dissemination of antimicrobial resistance genes within the gene pool to which staphylococci and other Firmicutes have access. This chapter is intended to provide an overview of the current knowledge of plasmid-mediated antimicrobial resistance in staphylococci and other Firmicutes.

Characterization of Mobile Staphylococcus equorum Plasmids Isolated from Fermented Seafood That Confer Lincomycin Resistance

The complete nucleotide sequences of lincomycin-resistance gene (lnuA)-containing plasmids in Staphylococcus equorum strains isolated from the high-salt-fermented seafood jeotgal were determined. These plasmids, designated pSELNU1–3, are 2638-bp long, have two polymorphic sites, and encode typical elements found in plasmids that replicate via a rolling-circle mechanism including the replication protein gene (rep), a double-stranded origin of replication, a single-stranded origin of replication, and counter-transcribed RNA sequence, as well as lnuA. Plasmid sequences exhibit over 83% identity to other Staphylococcus plasmids that harbor rep and lnuA genes. Further, three pairs of identified direct repeats may be involved in inter-plasmid recombination. One plasmid, pSELNU1, was successfully transferred to other Staphylococcus species, Enterococcus faecalis, and Tetragenococcus halophilus in vitro. Antibiotic susceptibility of the transconjugants was host-dependent, and transconjugants maintained a lincomycin resistance phenotype in the absence of selective pressure over 60 generations.

MRSA Pediatric clone expressing ermC plus lnuA genes causing nosocomial transmission and healthcare workers colonization in a neonatal intensive care unit

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of both nosocomial and community-acquired infections. We describe an outbreak caused by the MRSA Pediatric clone expressing an unusual lincosamide resistant phenotype. Between January and May 2006, an MRSA outbreak was detected at the Neonatal Unit of Hospital Interzonal General de Agudos "Evita", Buenos Aires Province, Argentina that affected ten patients. Seven isolates from seven patients plus five MRSA recovered from health care workers (nasal carriage) were studied. Two phenotypes were observed: (i) ELCi (10), resistance to erythromycin and lincomycin and inducible resistance to clindamycin; (ii) ELiCi (2), resistance to erythromycin and inducible resistance to lincomycin and clindamycin. All 12 MRSA were resistant to oxacillin, erythromycin and gentamicin. Isolates expressing the ELCi-phenotype showed lincomycin MIC values between 16 and 32mg/L, while the remaining 2 isolates with ELiCi-phenotype presented a MIC value of 0.5mg/L. No differences were observed between the clindamycin MIC values in both phenotypes, ranging 0.25-0.5mg/L. Isolates showing ELCi-phenotype harbored ermC plus lnuA genes, and the other two only ermC gene. All 12 isolates were genetically related and belonged to the Pediatric clone (ST100) harboring a new variant of SCCmecIV. This is the first MRSA outbreak expressing an unusual ELCi phenotype due to a combination of ermC plus lnuA genes.

Expansion of a plasmid classification system for Gram-positive bacteria and determination of the diversity of plasmids in Staphylococcus aureus strains of human, animal, and food origins

An expansion of a previously described plasmid classification was performed and used to reveal the plasmid content of a collection of 92 Staphylococcus aureus strains of different origins. rep genes of other genera were detected in Staphylococcus. S1 pulsed-field gel electrophoresis (PFGE) hybridizations were performed with 18 representative S. aureus strains, and a high number of plasmids of different sizes and organizations were detected.

Mechanisms of antimicrobial resistance and genetic relatedness among enterococci isolated from dogs and cats in the United States

AIMS

In this study, mechanisms of antimicrobial resistance and genetic relatedness among resistant enterococci from dogs and cats in the United States were determined.

METHODS AND RESULTS

Enterococci resistant to chloramphenicol, ciprofloxacin, erythromycin, gentamicin, kanamycin, streptomycin, lincomycin, quinupristin/dalfopristin and tetracycline were screened for the presence of 15 antimicrobial resistance genes. Five tetracycline resistance genes [tet(M), tet(O), tet(L), tet(S) and tet(U)] were detected with tet(M) accounting for approx. 60% (130/216) of tetracycline resistance; erm(B) was also widely distributed among 96% (43/45) of the erythromycin-resistant enterococci. Five aminoglycoside resistance genes were also detected among the kanamycin-resistant isolates with the majority of isolates (25/36; 69%) containing aph(3')-IIIa. The bifunctional aminoglycoside resistance gene, aac(6')-Ie-aph(2'')-Ia, was detected in gentamicin-resistant isolates and ant(6)-Ia in streptomycin-resistant isolates. The most common gene combination among enterococci from dogs (n = 11) was erm(B), aac(6')-Ie-aph(2'')-Ia, aph(3')-IIIa, tet(M), while tet(O), tet(L) were most common among cats (n = 18). Using pulsed-field gel electrophoresis (PFGE), isolates clustered according to enterococcal species, source and antimicrobial gene content and indistinguishable patterns were observed for some isolates from dogs and cats.

CONCLUSION

Enterococci from dogs and cats may be a source of antimicrobial resistance genes.

SIGNIFICANCE AND IMPACT OF THE STUDY

Dogs and cats may act as reservoirs of antimicrobial resistance genes that can be transferred from pets to people. Although host-specific ecovars of enterococcal species have been described, identical PFGE patterns suggest that enterococcal strains may be exchanged between these two animal species.

Antimicrobial susceptibility and invasive ability of Staphylococcus aureus isolates from mastitis from dairy backyard systems

Fifteen (15) backyard farms were investigated to determine the antimicrobial susceptibility and invasion ability of S. aureus isolates from cows with subclinical mastitis in México. A total of 106 cows were sampled and 31 S. aureus isolates were recovered. S. aureus isolates were resistant to penicillin class antibiotics and susceptible to gentamicin and cetyltrimethylammonium bromide. STA9 and STA13 isolates were resistant to erythromycin (MIC > 25 microg/ml) and lincomycin (STA13, MIC > 25 microg/ml; STA9, MIC > 100 microg/ml). STA9 isolate harbors the erm(B) and msr(A) genes, whereas STA13 isolate harbors the erm(C) gene. STA9 and STA13 isolates contains the lnu(A) gene. Only 5 isolates (STA11, STA13, STA14, STA15 and STA21) were able to internalize in bovine mammary epithelial cells. These results indicate that S. aureus isolates from dairy backyard farms showed differences in the antimicrobial susceptibility patterns and invasion ability in bovine mammary epithelial cells. This kind of evaluations should be performed in different dairy regions, since resistance patterns and isolate diversity vary on a per-region basis.

Identification and characterization of nine novel types of small staphylococcal plasmids carrying the lincosamide nucleotidyltransferase gene lnu(A)

OBJECTIVES

To date, very little is known about lincosamide resistance plasmids in staphylococci with only a single lnu(A)-carrying staphylococcal plasmid having been sequenced completely. The aim of this study was to characterize small lnu(A)-carrying plasmids isolated from bovine coagulase-negative staphylococci (CoNS).

METHODS

Nine CoNS isolates with MICs of the lincosamide pirlimycin of 1-4 mg/L were tested for the presence of the lnu(A) gene. Its location was determined by Southern-blot hybridization. The lnu(A)-carrying plasmids were transformed into Staphylococcus aureus RN4220 and compared by restriction mapping and subsequent sequencing. Selected plasmids were investigated for their copy number and their lnu(A) gene expression via RT real-time PCR.

RESULTS

The lnu(A) gene was detected on plasmids in all isolates. Sequence analysis revealed that these plasmids carried a rep gene, coding for the replication initiator protein, and the resistance gene lnu(A), coding for a lincosamide nucleotidyltransferase. While the Lnu(A) proteins were closely related (91.3-100% amino acid identity), the Rep proteins differed distinctly (27.4-100% amino acid identity), but showed similarity (81.4-98.5%) to Rep proteins of other small staphylococcal resistance plasmids. Sequence features of rolling-circle plasmids, such as the single-strand (ssoA) and double-strand (dso) origins of replication, were identified. For two plasmid types detected, the lincosamide resistance level varied with regard to the amounts of lnu(A) transcripts detected.

CONCLUSIONS

Structurally different lnu(A)-carrying plasmids were detected in various CoNS species. The detection of the same lnu(A) gene in different plasmid backbones suggested the exchange of the gene via interplasmid recombinational events.

Invasive potential of bacterial isolates associated with subclinical bovine mastitis

This work describes differences in the invasive ability of bacterial isolates associated with mastitis. Invasion ability was determined by the uptake and survival in a primary culture of bovine mammary epithelial cells (BMEC). BMEC were isolated from a healthy lactating cow and characterized by their morphology, immunostaining for cytokeratin and the detection of beta- and kappa-casein mRNAs. Ten bacterial isolates comprising the staphylococcal species Staphylococcus aureus (3), Staphylococcus epidermidis (1), Staphylococcus haemolyticus (1), Staphylococcus equorum (2), Staphylococcus xylosus (1) and Brevibacterium stationis (2) obtained from raw milk of cows with mastitis from backyard farms were assayed for their ability to invade BMEC. Only two S. aureus and one S. epidermidis isolates were able to invade BMEC, at similar levels to the S. aureus control strain ATCC 27543. In conclusion, using the in vitro model of infection used in this study, differences in bacterial invasion capability may be detected.

Antimicrobial resistance of coagulase-negative staphylococci from bovine subclinical mastitis with particular reference to macrolide–lincosamide resistance phenotypes and genotypes

OBJECTIVES

The aim of this study was to analyse coagulase-negative staphylococci (CoNS) for their resistance to antimicrobial agents approved for the control of pathogens involved in bovine mastitis, with particular reference to macrolide and/or lincosamide (ML) resistance and the resistance genes involved.

METHODS

A total of 298 CoNS collected between 2003 and 2005 in Germany from cases of subclinical mastitis in dairy cows were identified to the species level and investigated for their MICs by broth microdilution. ML-resistant isolates were subjected to plasmid profiling and electrotransformation experiments. The ML resistance genes were detected using PCR and hybridization. Selected PCR products were cloned and sequenced.

RESULTS

The CoNS isolates used in this study showed a low level of resistance to all antimicrobial agents tested (0-7.4%) except ampicillin (18.1%). In the erythromycin-resistant and/or pirlimycin-resistant isolates, the ML resistance genes erm(B), erm(C), msr(A), mph(C) and lnu(A) were present, either alone or in different combinations. Isolates carrying erm methylase genes or the exporter gene msr(A) showed higher MICs than those harbouring only the genes mph(C) or lnu(A) coding for inactivating enzymes. Most of the ML resistance genes were found on plasmids.

CONCLUSIONS

This is the first report of pirlimycin MICs for CoNS collected from cases of bovine subclinical mastitis in Germany. After 3-5 years of veterinary therapeutic use, pirlimycin resistance was rarely detected among CoNS. The finding that five different resistance genes--present in various combinations--were responsible for ML resistance underlines the heterogeneous character of this resistance trait.

The plasmid pBMBt1 from Bacillus thuringiensis subsp. darmstadiensis (INTA Mo14-4) replicates by the rolling-circle mechanism and encodes a novel insecticidal crystal protein-like gene

This work describes a novel rolling-circle replicating (RCR) plasmid pBMBt1 from Bacillus thuringiensis subsp. darmstadiensis (INTA Mo14-4) encoding an insecticidal crystal protein-like gene. pBMBt1 (6700 bp) contains three ORFs and their putative transcription initiation sites and Shine-Dalgarno sequences were localized. ORF1 encodes a 34.6 kDa protein which showed identity with the protein CryC53 from B. thuringiensis subsp. cameroun (24.6%), the Cry15Aa insecticidal crystal protein from B. thuringiensis subsp. thompsoni (21.9%) and the Mtx3 protein from Bacillus sphaericus (27.8%). The ORF2 (52.3 kDa) showed a 74% identity with the Mob protein coded by pUIBI-1 from B. thuringiensis subsp. entomocidus and 64% identity with the Mob protein of pBMY1 from Bacillus mycoides; both Mob proteins belong to the pMV158 superfamily. To evaluate the Mob protein, the plasmid pHTMob14-4 was constructed. This plasmid shows transfer frequencies of 9.1x10(-6) in B. thuringiensis subsp. israelensis (4Q7Gm(R)). The ORF3 (23.6 kDa) gene product is homologous to the Rep protein from the plasmid pBMYdx of B. mycoides (37.6%). A putative double-strand origin with significant homology to that of B. thuringiensis plasmids, and an ssoA-type single-strand origin were also identified. Detection of single-stranded pBMBt1 DNA replicating intermediaries suggests that replication occurs via the rolling-circle mechanism.

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.