Reduced vancomycin susceptibility in porcine ST9 MRSA isolates

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.

Single-nucleotide polymorphisms in a vancomycin-resistant Staphylococcus aureus strain based on whole-genome sequencing

The emergence of vancomycin-resistant Staphylococcus aureus (VRSA) threatens global health. The mechanism of vancomycin resistance of VRSA without vanA gene acquisition was not fully elucidated. Therefore, we aimed to determine the mechanism of vancomycin resistance of VRSA besides that by vanA gene acquisition. In this study, we obtained vancomycin-resistant strains (V036-V64; MIC = 64 µg /ml) from susceptible strain (V036; MIC = 0.5 µg /ml) by exposure of vancomycin in vitro and examined the phenotypic characteristics and antibiotic susceptibility profiles of the resistant strain (V036-V64). To identify the genetic variations caused vancomycin resistance, we determined the complete genome sequences of V036 and V036-V64 and analyzed for single-nucleotide polymorphisms (SNPs) between two strains. Morphologically, V036-V64 had a twofold thicker cell wall compared with V036. Linezolid, rifampicin, and ceftaroline had similar MIC ranges against V036-V64 and V036, but V036-V64 showed lower susceptibilities to daptomycin and telavancin. We detected eight single-nucleotide polymorphisms differing between V036-V64 and V036: rimM (G16D), ssaA2 (G128A), rpsK (P60R), rpoB (R917C), walK (T492R), d-alanyl-d-alanine carboxypeptidase (L307I), vraT (A152V), and chromosome segregation ATPase (T440I). This study demonstrates that, under selective pressure, by the accumulation of mutations in genes related to cell wall synthesis, vancomycin-susceptible S. aureus can develop thicker cell walls and, hence, develop high vancomycin resistance. Thus, we highlight a novel vanA-negative mechanism for VRSA emergence.

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.

Vancomycin Resistance in Staphylococcus aureus

The evolution of Staphylococcus aureus during the modern antibiotic era has been delineated by distinct strain emergence events, many of which include acquisition of antibiotic resistance. The relative high burden of methicillin-resistant S. aureus (MRSA) in healthcare and community settings is a major concern worldwide. Vancomycin, a glycopeptide antibiotic that inhibits cell wall biosynthesis, remains a drug of choice for treatment of severe MRSA infections. S. aureus strains exhibiting increased resistance to vancomycin, known as vancomycin intermediate-resistant S. aureus (VISA) (MIC = 4-8 µg/mL), were discovered in the 1990s. The molecular basis of resistance in VISA is polygenic and involves stepwise mutations in genes encoding molecules predominantly involved in cell envelope biosynthesis. S. aureus isolates with complete resistance to vancomycin (MIC ≥ 16 µg/mL) are termed vancomycin-resistant S. aureus (VRSA)-they were first reported in the U.S. in 2002. Resistance in VRSA is conferred by the vanA gene and operon, which is present on a plasmid. Although treatment of VRSA infections is challenging, the total number of human VRSA infections to date is limited (14 in the U.S.). By comparison, the burden of VISA is relatively high and the molecular mechanisms of resistance are less well-defined. VISA are associated with persistent infections, vancomycin treatment failure, and poor clinical outcomes. Here, we review in brief progress made toward understanding the acquisition of antibiotic resistance in S. aureus, with an emphasis on the molecular mechanisms underlying vancomycin resistance.

Livestock-associated meticillin-resistant Staphylococcus aureus in Asia: an emerging issue?

In addition to being a human pathogen, Staphylococcus aureus causes an array of infections in economically important livestock animals, particularly pigs. In Asia, there have been few reports on livestock-associated meticillin-resistant S. aureus (LA-MRSA), mostly from developed countries, with very few data available from resource-limited countries, not because of low prevalence but probably due to a shortage of diagnostic facilities. Unlike the wide spread of sequence type 398 (ST398) LA-MRSA in European countries and North America, ST9 predominates in most Asian countries. The prevalence of LA-MRSA among pigs in Asian countries varied widely (0.9-42.5%). The prevalence may vary by geographic location, age of pigs and sampling methodologies. Among pig farmers, the prevalence of nasal MRSA colonisation varied from 5.5% in Malaysia to 15% in China and 19.2% in Taiwan. Although most LA-MRSA isolates in Asia are of the same ST, molecular characteristics are not all the same. Dominant isolates in China were characterised as spa type t899-SCCmec III and t899-SCCmec IVb or V for isolates in Hong Kong, and t899-untypeable SCCmec for Taiwan. Dominant isolates in Malaysia were spa type t4358-SCCmec V and t337-SCCmec IX for isolates in Thailand. In addition, MRSA ST221 was reported in Japan and MRSA ST398 was isolated from commercial pigs in South Korea. Attention should be paid because pigs could become an important reservoir for MRSA and spread them to humans, as observed in many countries. There is a potential risk from the livestock reservoir to community and hospitals.