Plasmid-encoded resistance to macrolides and lincosamides in Staphylococcus hyicus

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.

In vitro assessment of properties associated with the survival through the gastro-intestinal tract of staphylococci isolated from traditional sausage fermentation

Thirteen Staphylococcus sp. strains, previously isolated from spontaneous sausage fermentation, were in vitro examined for properties associated with their ability to survive through the gastro-intestinal tract. None of the strains were able to survive exposure to pH 1 or pH 2, while for most of them, a population reduction, ranging from 77.3% to 99.0% and a surviving population from 1.7 x 10(8) to 9.0 x 10(6) was observed after exposure to pH 3. None of the strains exhibited bile salt hydrolase activity or production of antimicrobial compounds, while all of them were resistant to pancreatin. Only S. cohnii cohnii LQC 5112 was found to be alpha-haemolytic, seven other strains were beta-haemolytic and the rest gamma-haemolytic. All strains were sensitive to erythromycin, ampicillin (but S. intermedius LQC 5023) and chloramphenicol while most of them were sensitive to tetracycline. On the other hand, most of the strains were resistant to novobiocin. Furthermore, their aptitude, not only to withstand, but to proliferate in the presence of bile salts, as well, even at an acidic environment and their ability to adhere to stainless-steel plates, indicate the need for an in vivo study.

Molecular characterization of the erythromycin resistance plasmid pPV142 from Staphylococcus simulans

The 2.5-kb erythromycin resistance (EmR) plasmid pPV142 of Staphylococcus simulans 13044 was isolated and characterized. Sequence analysis identified ORF1 and ORF2 encoding a 158-residue replication protein (Rep142) and a 244-residue erythromycin resistance protein (Erm, rRNA adenine N-6-methyltransferase), respectively. Structural analysis and Southern hybridization showed that the rep and ermM genes in pPV142 shared homology with the EmR plasmid pPV141 (2.4 kb) of S. chromogenes 3688 and other EmR plasmids known to exist in staphylococci and bacilli. Based on the presence of a 61-bp repeat upstream of the ermM gene, pPV142 is apparently a unique member of the pSN2 family of EmR plasmid able to express erythromycin resistance constitutively.

Molecular properties of the erythromycin resistance plasmid pPV141 from Staphylococcus chromogenes

The 2.3-kb erythromycin resistance (EmR) plasmid pPV141 of Staphylococcus chromogenes 3688 was isolated and characterized. Nucleotide sequence analysis identified ORF1 and ORF2 separated by a 445-bp spacing, encoding a 158-residue replication protein (Rep141) and a 244-residue erythromycin resistance protein (Erm, rRNA adenine N-6-methyltransferase), respectively. Structural analysis and Southern hybridization showed that the rep and ermM genes in pPV141 shared homology with other known EmR plasmids. Based on sequence analysis, pPV141 was classified as a unique member of the pSN2 family of EmR plasmids.

Antimicrobial susceptibility of Staphylococcus hyicus isolated from exudative epidermitis in pigs

Exudative epidermitis or greasy pig syndrome is caused by the coagulase-variable staphylococcal species Staphylococcus hyicus. Treatment of this disease is problematic because of the limited number of antimicrobial agents available for this purpose. Thirteen antimicrobial agents were evaluated for their activities against 100 S. hyicus strains isolated from pigs with exudative epidermitis. Novobiocin was the most active compound tested, with an MIC for 90% of the strains tested (MIC90) of < or = 0.06 microgram/ml. Enrofloxacin, ampicillin, and ceftiofur were the next most active compounds, with MIC90s of 0.25, 0.5, and 1.0 microgram/ml, respectively. However, 41.4% of the 99 strains tested were positive for beta-lactamase production. The MIC90s of erythromycin, tetracycline, and streptomycin were > 32.0 micrograms/ml. Initial testing with sulfadiazine-trimethoprim yielded an MIC90 of > 64.0 micrograms/ml, but subsequent testing with thymidine phosphorylase-supplemented medium yielded an MIC90 of 0.06 microgram/ml. Both lincomycin and spectinomycin were relatively inactive against the S. hyicus strains tested, with MIC90s of > 64.0 and > 128.0 micrograms/ml, respectively. However, the combination of the two compounds at ratios of 1:2 (lincomycin to spectinomycin) and 1:8 were more active, with MIC90s of 16.0 and 4.0 micrograms/ml, respectively. These results indicate that novobiocin and sulfadiazine-trimethoprim were the most active compounds tested against the S. hyicus strains isolated from pigs with exudative epidermitis. Furthermore, the combination of lincomycin and spectinomycin was more active than the individual compounds against the strains tested.

Antibiotic-resistance and plasmids in Staphylococcus hyicus isolated from pigs with exudative epidermitis and from healthy pigs

A total of 100 S. hyicus strains isolated from healthy piglets and piglets with exudative epidermitis originating from 100 different herds was examined for drug-resistance and prevalence of plasmids. Resistance to macrolide/linosamide antibiotics could be related to plasmids in 55 (93%) of the 59 resistant strains: A plasmid of 2.4 kb mediating resistance to macrolides and lincosamides was observed in 25 strains, and a plasmid of 11.5 kb mediating resistance to both macrolides/lincosamides and tetracycline was observed in 30 strains. A plasmid with a molecular weight of 4.5 kb was shown by curing experiments to be associated with resistance to tetracycline in 12 strains. All together, 47 strains were resistant to tetracycline. In 42 (89%) of these strains tetracycline-resistance was found to be encoded by plasmids. Fifty six strains were resistant to streptomycin, and resistance was associated with the presence of a 4.4 kb plasmid in 17 strains studied. Resistance to penicillin, observed in 44 strains, and resistance to kanamycin, observed in 15 strains, could not be related to plasmids in any of these strains. The 11.5 kb plasmid was observed in 39% of the strains isolated from piglets with EE, and in 7% of the strains isolated from healthy piglets. Despite its higher prevalence in strains from piglets with EE, the 11.5 kb plasmid could not be shown to encode production of capsule or exfoliative substances: factors which might play a role in the development of exudative epidermitis in piglets.