Influence of Immune Status on the Airborne Colonization of Piglets with Methicillin-Resistant Staphylococcus aureus (MRSA) Clonal Complex (CC) 398

Colonized vertebrates including humans and pigs are to date the main reservoirs of livestock-associated Methicillin-resistant Staphylococcus aureus (LA-MRSA). Currently, the mechanisms underlying colonization of pigs are not fully understood. We investigated the influence of piglet pre-immune status on airborne MRSA colonization. Three groups of MRSA-negative piglets were primed and exposed to airborne LA-MRSA (104 colony forming units (cfu)/m3) in an aerosol chamber for 24 h. One group was treated intramuscularly with dexamethasone (1 mg/kg body weight) to imitate weaning stress. The second group was exposed to bacterial endotoxin containing MRSA aerosol. Both conditions play a role in the development of multifactorial diseases and may promote MRSA colonization success. The third group served as control. The piglets' MRSA status was monitored for 21 days via swab samples. At necropsy, specific tissues and organs were analyzed. Blood was collected to examine specific immunological parameters. The duration of MRSA colonization was not extended in both treated groups compared to the control group, indicating the two immune-status influencing factors do not promote MRSA colonization. Blood sample analysis confirmed a mild dexamethasone-induced immune suppression and typical endotoxin-related changes in peripheral blood. Of note, the low-dose dexamethasone treatment showed a trend of increased MRSA clearance

A longitudinal study on the persistence of Livestock Associated-MRSA in swine herds

In recent years, a new type of MRSA, now called livestock-associated MRSA (LA-MRSA), belonging to the clonal complex (CC) 398, has globally emerged in swine world wide. Aim of this study was to gain more insight into the persistence of LA-MRSA in different types of pig farms over a period of two years. To investigate this, 15 MRSA-positive herds from a previous study were selected; an additional pig farm was selected because this one was found to be positive with a human MRSA-strain. Starting in September 2009, five dust samples were collected every two months. Samples were analysed for MRSA and spa-typing was done to confirm that MRSA isolates belonged to CC398 and to gain insight into persistence of strains within a farm. Three herds were positive on all sampling occasions and were consistently contaminated with LA-MRSA. In the remaining herds, occasionally no positive dust samples were found at some sampling moments. The predominating spa -types were t011 and t108. A maximum of 5 different spa-types were found in two herds, with 3 different spa-types present in one sampling, indicating multiple introductions. These results show that LA-MRSA remains present on a pig farm over a long period. Most likely, transmission within the herd occurs after initial introduction, and an endemic situation seems to be the endpoint. The relatively low sensitivity of dust sampling compared to sampling of animals, the small sample size and lack of strict standardization of dust sampling might explain occasional negative samplings in overall positive herds. However, a true change of a positive MRSA-status to a negative status, followed by re-introduction cannot be ruled out in our study design

Super Staph in the Community: Is It Evolving?

Staphylococcus aureus infections are a common cause of disease, particularly in colonized people. They frequently cause staph infections and are often dubbed “Super Staph” because they are virulent and multidrug resistant. Recently, a series of published articles have reported that community-acquired methicillin-resistant S. aureus (CA-MRSA) strains are evolving and becoming more prevalent in households. In contrast, health care acquired MRSA (HA-MRSA) is declining in the United States. The changing “Superbugs” have often been used as an example of “evolution in action.” Although MRSA infections have become more prevalent in the community, studies of college students carrying S. aureus and MRSA colonization are lacking. In early studies at Liberty University, we have found that students in microbiology classes who have more contact with individuals in a healthcare setting are more likely to carry MRSA in their body. The classes that had the highest rate of HA-MRSA carriage were those primarily populated by nursing students. Nursing students typically have greater exposure to clinical settings and nursing homes than students in other fields of study. However, in research collected this past year, 2014–2015, we observed a shift to students of many majors now carrying CA-MRSA. At Liberty University, we sampled 544 students and had up to 20%+ MRSA rates common among clinically oriented students, five to ten times the national average. We have seen a changing profile from HA-MRSA to CA-MRSA; this change has the potential to be dangerous, since the new strains are more virulent and aggressive. CA-MRSA is somewhat difficult to define, but is mostly associated with antibiotic profile, toxin genes, and place of acquisition. There is a variation of S. aureusstrains, but most change is found in tightly knit groups: households, dorms, and other close living quarters. The bacteria are “ping-ponging” around among students, changing as they go. This change is real and clearly indicates an emergence of new MRSA variants that some may call microevolution. It is, however, not Darwinian upward-onward evolution but clearly adaptive changes within a species: variants on a theme. MRSA strains are acquiring more genes as they “ping-pong” from one person to another. They change their virulence as they pick up more foreign genes (via phage or plasmids) and vary as they go. The purpose of this article is to provide a reasonable explanation for the genesis, emergence, and the new dominance of Community-associated (CA) MRSA. It also addresses the issue of whether this phenomenon is “evolution in action.” Microbiology research based on the creation paradigm appears to provide some answers to these puzzling questions regarding the new variants of Staphylococcus aureus and its emerging dominance in the United States

Bicarbonate Resensitization of Methicillin-Resistant Staphylococcus aureus to β-Lactam Antibiotics.

Endovascular infections caused by methicillin-resistant Staphylococcus aureus (MRSA) are a major health care concern, especially infective endocarditis (IE). Standard antimicrobial susceptibility testing (AST) defines most MRSA strains as "resistant" to β-lactams, often leading to the use of costly and/or toxic treatment regimens. In this investigation, five prototype MRSA strains, representing the range of genotypes in current clinical circulation, were studied. We identified two distinct MRSA phenotypes upon AST using standard media, with or without sodium bicarbonate (NaHCO3) supplementation: one highly susceptible to the antistaphylococcal β-lactams oxacillin and cefazolin (NaHCO3 responsive) and one resistant to such agents (NaHCO3 nonresponsive). These phenotypes accurately predicted clearance profiles of MRSA from target tissues in experimental MRSA IE treated with each β-lactam. Mechanistically, NaHCO3 reduced the expression of two key genes involved in the MRSA phenotype, mecA and sarA, leading to decreased production of penicillin-binding protein 2a (that mediates methicillin resistance), in NaHCO3-responsive (but not in NaHCO3-nonresponsive) strains. Moreover, both cefazolin and oxacillin synergistically killed NaHCO3-responsive strains in the presence of the host defense antimicrobial peptide (LL-37) in NaHCO3-supplemented media. These findings suggest that AST of MRSA strains in NaHCO3-containing media may potentially identify infections caused by NaHCO3-responsive strains that are appropriate for β-lactam therapy

Prevalence and Antimicrobial Agent Susceptibility of Methicillin-resistant Staphylococcus aureus in Healthy Pediatric Outpatients in Las Vegas

Colonization and infection by community-associated resistant strains of Staphylococcus aureus are being reported in epidemic proportions. The purpose of this study was to determine the local prevalence of methicillin-resistant Staphylococcus aureus (MRSA) colonization in children and to characterize the MRSA isolates in the laboratory with regard to antimicrobial agent susceptibility patterns, and the presence of the mecA and the Panton-Valentine leukocidin (PVL) genes. Nasal swabs were collected at two pediatric clinics from a total of 505 children during health maintenance visits. A brief questionnaire was administered to collect demographic data and pertinent medical, family, and social history. Samples were cultured onto 2 selective media for S. aureus and MRSA. Potential MRSA isolates were further evaluated by real-time polymerase chain reaction (PCR), and for susceptibility to eight antibiotics by disk diffusion. Culture results showed that MRSA was present in 15 of the 505 specimens (3.0%). Six different antimicrobial susceptibility profiles were observed among the MRSA isolates. PCR amplification results showed that all 15 MRSA isolates were positive for the presence of the mecA gene, and 10 MRSA isolates contained the PVL gene. Understanding local prevalence rates and the role of colonization in infection are needed to develop effective interventions to reduce MRSA infections

Transmission dynamics of methicillin-resistant Staphylococcus aureus in pigs

From the mid-2000s on, numerous studies have shown that methicillin-resistant Staphylococcus aureus (MRSA), renowned as human pathogen, has a reservoir in pigs and other livestock. In Europe and North America, clonal complex (CC) 398 appears to be the predominant lineage involved. Especially worrisome is its capacity to contaminate humans in close contact with affected animals. Indeed, the typical multi-resistant phenotype of MRSA CC398 and its observed ability of easily acquiring genetic material suggests that MRSA CC398 strains with an increased virulence potential may emerge, for which few therapeutic options would remain. This questions the need to implement interventions to control the presence and spread of MRSA CC398 among pigs. MRSA CC398 shows a high but not fully understood transmission potential in the pig population and is able to persist within that population. Although direct contact is probably the main route for MRSA transmission between pigs, also environmental contamination, the presence of other livestock, the herd size, and farm management are factors that may be involved in the dissemination of MRSA CC398. The current review aims at summarizing the research that has so far been done on the transmission dynamics and risk factors for introduction and persistence of MRSA CC398 in farms

Genetic Variation in Spatio-Temporal Confined USA300 Community-Associated MRSA Isolates: A Shift from Clonal Dispersion to Genetic Evolution?

NTRODUCTION: Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) are increasingly isolated, with USA300-0114 being the predominant clone in the USA. Comparative whole genome sequencing of USA300 isolates collected in 2002, 2003 and 2005 showed a limited number of single nucleotide polymorphisms and regions of difference. This suggests that USA300 has undergone rapid clonal expansion without great genomic diversification. However, whole genome comparison of CA-MRSA has been limited to isolates belonging to USA300. The aim of this study was to compare the genetic repertoire of different CA-MRSA clones with that of HA-MRSA from the USA and Europe through comparative genomic hybridization (CGH) to identify genetic clues that may explain the successful and rapid emergence of CA-MRSA. MATERIALS AND METHODS: Hierarchical clustering based on CGH of 48 MRSA isolates from the community and nosocomial infections from Europe and the USA revealed dispersed clustering of the 19 CA-MRSA isolates. This means that these 19 CA-MRSA isolates do not share a unique genetic make-up. Only the PVL genes were commonly present in all CA-MRSA isolates. However, 10 genes were variably present among 14 USA300 isolates. Most of these genes were present on mobile elements. CONCLUSION: The genetic variation present among the 14 USA300 isolates is remarkable considering the fact that the isolates were recovered within one month and originated from a confined geographic area, suggesting continuous evolution of this clone

Prevalence and Antimicrobial Susceptibility of Methicillin-resistant Staphylococcus aureus in Pregnant Women and Their Newborns in Las Vegas, Nevada

Colonization and infection by resistant strains of Staphylococcus aureus are being reported in epidemic proportions. The goal of this study was to determine the local prevalence of methicillin-resistant Staphylococcus aureus (MRSA) colonization in pregnant women in southern Nevada and how it correlates with colonization and infection of their neonates. Signed consent was obtained, and a brief questionnaire was administered by the medical staff to each pregnant woman to collect demographic data and pertinent medical, family and social history. Nasal and vaginal specimens were obtained from pregnant women at ≥35 weeks gestation, and nasal and umbilicus specimens were obtained from their newborns. Specimens were cultured onto two selective media for S. aureus and MRSA. Potential MRSA isolates were further evaluated for susceptibility to antibiotics. Specimens from 307 pregnant women and 174 neonates were collected, resulting in 172 mother-neonate paired specimens. A total of 278 questionnaires were received from study participants. MRSA prevalence in pregnant women was 1.0% and 0.3% for nasal and vaginal specimens, respectively. The MRSA prevalence in neonates was 0% and 0.6% for nasal and umbilical specimens, respectively. Four different antimicrobial susceptibility profiles were observed among the MRSA isolates. The results did not show transmission of MRSA from pregnant women to their newborns, or infections of newborns with MRSA. It is expected that the results of this study will inform future decisions on surveillance, treatment and prevention of MRSA infections in Nevada

Detection of mecC-Methicillin-resistant Staphylococcus aureus isolates in river water : a potential role for water in the environmental dissemination

Methicillin-resistant Staphylococcus aureus (MRSA) is a public health concern due to limited treatment options. The recent description of a mecA homologue, mecC in human and cattle, led to studies to detect this new variant in human and other animal species. Detection of mecC in wild boar and fallow deer in a Spanish game estate led us to further investigate the presence of mecC-MRSA at this location. Samples from cattle, wild animals, workers and river water were tested. A further three mecC-MRSA isolates were obtained from river water. Molecular characterization (multilocus sequence typing and spa typing) and antimicrobial susceptibility testing (broth microdilution) showed that isolates were similar to those detected in wild animals. Whole genome sequencing confirmed that the isolates from the river water and wild animals in the same geographic area were all closely related isolates of ST425 mecC-MRSA. The presence of mecC-MRSA in the river water highlights the potential role of water in the dissemination of mecC-MRSA