MDR-Acinetobacter baumannii beats MRSA in the war for ICU predominance

February 14, 2013 Jon Otter (@jonotter) Acinetobacter, Contamination, MRSA Acinetobacter, contamination, ICU, MRSA, transmission, virulence

A. baumannii is a notorious nosocomial pathogen due to a combination of its environmental resilience, its association with antimicrobial resistance and its outbreak potential. Colonized patients and contaminated environments are thought to be the primary reservoirs for the nosocomial transmission of this pathogen.

A recent study from China suggests that carriers of MDR A. baumannii (MDR-AB) show stronger ability to contaminate their immediate environment than those carrying MRSA and that MDR-AB spreads more easily and rapidly among inpatients compared with MRSA. The 20-month study was conducted in a respiratory ICU (RICU) where active screening of patients and targeted environmental screening for MRSA and MDR-AB were performed. The environmental samples were collected from 6 sites on patients’ bed linens.

High levels of carriage and nosocomial acquisition were found among the 175 patients admitted to the RICU where 44% of the patients were MDR-AB positive (80% of which were hospital acquired) and 24% of patients were MRSA carriers (60% of which were hospital acquired). Interestingly, 15.4% of the patients were co-carriers of MRSA and MDR-AB.

Researchers found that bed linens were commonly contaminated with MRSA and MDR-AB and that the contamination rate for MDR-AB was significantly higher than that of MRSA. Of the 576 MRSA samples, 26.6% were positive, and 51.6% of the 1,176 MDR-AB swabs were positive. This is surprising given the strict daily extensive cleaning practices, thrice daily bed linen changes and stringent terminal sterilization immediately after discharge of carriers. Researchers used the weekly colonisation pressure adjusted by degree of bed linen contamination (WCPe) and weekly acquisition rate (WAR) as parameters to evaluate the potential spread of these pathogens among inpatients. They found a positive significant correlation between the WCPe and WAR values for both organisms but both the WCPe and WAR of MDR-AB were significantly higher than for MRSA.

This study shows that environmental contamination with MDR-AB and the rate of its nosocomial acquisition is significantly higher than those for MRSA, which may explain why MDR-AB is able to spread among inpatients more rapidly. Although the study found positive significant correlation between the WCPe and WAR in the subsequent weeks, this correlation does not necessarily indicate causality. Nevertheless, the authors conclude that reduction of environmental contamination close to MDR-AB positive patients is crucial in controlling MDR-AB transmission.

Article citation:

Sui W, Wang J, Wang H et al. Comparing the transmission potential of Methicillin-resistantStaphylococcus aureus and multidrug-resistant Acinetobacter baumannii among inpatients using target environmental monitoring. Am J Infect Control. 2012. doi: 10.1016/j.ajic.2012.08.007

Hydrogen peroxide at war with catalase

January 17, 2013 Jon Otter (@jonotter) Disinfection, MRSA catalase, disinfection, MRSA

A study published by the UK Health Protection Agency highlights the fact that MRSA dried on surfaces appears to be less susceptible to hydrogen peroxide vapour (HPV) than bacterial endospores in the form of commercially produced biological indicators. This is a surprise because bacterial endospores are generally considered to be close to the top of the tree in terms of resistance to disinfection and sterilization. The answer lies in the enzyme catalase. Catalase breaks down hydrogen peroxide and is produced by MRSA but not by the metabolically inert spores. A previous study also showed that catalase-producing bacteria were less susceptible to HPV than bacteria that did not produce catalase and metabolically inert spores.

Bacterial endospore biological indicators (BIs) are typically used to monitor the efficacy of HPV systems. Does the finding that MRSA and probably other catalase-producing bacteria are less susceptible to HPV, “cell for spore”, than bacterial endospores challenge the use of BIs to monitor decontamination using HPV? In vitro susceptibility is important, but the resistance of a ‘system’ to HPV will be determined by a number of factors including the relative susceptibility of the organisms, suspending medium, substrate and inoculum. Thus, you’ll never get a “one-size-fits-all” monitoring system for HPV. BIs provide a stringent, repeatable, consistent and safe method to monitor the efficacy of HPV. Whilst they may not be top of the tree in terms of efficacy, BIs provide a useful yard-stick for monitoring efficacy.

Article citation: Pottage T, Macken S, Walker JT, Bennett AM. Meticillin-resistant Staphylococcus aureus is more resistant to vaporized hydrogen peroxide than commercial Geobacillus stearothermophilus biological indicators. J Hosp Infect 2012; 80:41-5.

It’s time to give MRSA the red card

December 19, 2012 Jon Otter (@jonotter) MRSA bloodstream infection, Europe, MRSA

A remarkable study published by a German group found that countries whose national football team performed badly on a fair play indicator had a higher proportion of methicillin-resistance amongst bloodstream isolates. The team investigated the countries who qualified for the 2008 European Football Championship and gave each a ‘fair play indicator’ score (red or yellow cards / 100 min). They then used methicillin resistance data for S. aureus from the European Antimicrobial Resistance Surveillance System (EARSS) programme to plot against the fair play indicator score. The proportion of S. aureus resistant to methicillin correlated with the fair play indicator score (correlation coefficient of 0.632, p=0.038). Greece, Turkey, Italy and Romania clustered together with a high proportion of MRSA and a poor fair play indicator score. The Netherlands and Sweden had a low MRSA rate and a better fair play indicator score.

Does this study highlight cultural differences that influence the national rate of MRSA? Or is this a statistical fluke? Repeating the approach on past or future European challenges (or perhaps even broadening the net to the Football World Cup) will help to confirm the association. But either way, it’s a great study for a football fan!

Article citation: Meyer E, Gastmeier P, Schwab F. National MRSA rates run along with fair play of national football teams: a cross-national data analysis of the European Football Championship, 2008. Infection 2012 Aug 5. [Epub ahead of print]

What is “community-associated” MRSA?

December 6, 2012 Jon Otter (@jonotter) MRSA CA-MRSA, community, HA-MRSA, nosocomial

A study in this month’s ICHE highlights the problems with using epidemiological definitions to designate MRSA as “nosocomial”. The study evaluated the impact of different numerators and denominators on the rate of apparent hospital-onset MRSA across 32 hospitals in California. The time that patients were hospitalized before being considered hospital-onset varied from 48 to more than three days and denominators were also variable. The particular combination of numerator and denominator used resulted in significant differences in the proportion of MRSA cases designated hospital-onset. This has clear implications for comparing rates of hospital-attributable MRSA in the era of public reporting.

The paper raises a wider problem of how to define healthcare- and community-associated MRSA in the era of CA-MRSA strains as a cause of healthcare-associated infections. A recent review in JHI (Otter & French 2012) made the case for a genotypic definition of CA-MRSA. Epidemiological definitions were useful for differentiating CA-MRSA and HA-MRSA strain types in the past. However, although HA-MRSA strain types are rarely transmitted in the community, CA-MRSA strains have now begun to be transmitted in healthcare facilities, so epidemiological definitions are breaking down. CA-MRSA are community strains of S. aureus that have acquired mecA. They are distinct from HA-MRSA and should be defined genetically. Carriage of the Panton-Valentine leukocidin (PVL) or antimicrobial susceptibly profiles can be useful indicators of CA-MRSA but should not be used to define them. For the full assessment of their epidemiology, MRSA infections should now be characterised as (1) caused by HA- or CA-MRSA strain types; (2) acquired in community or healthcare settings; and (3) onset in the community or healthcare facility. (This review made the 10 ten list of the JHI Editors choice and is freely available online here.)

Article citations:

Datta R, Kuo King M, Kim D et al. What Is Nosocomial? Large Variation in Hospital Choice of Numerators and Denominators Affects Rates of Hospital-Onset Methicillin-Resistant Staphylococcus aureus. Infect Control Hosp Epidemiol 2012; 33: 1166-9.

Otter JA, French GL. Community-associated meticillin-resistant Staphylococcus aureus: the case for a genotypic definition. J Hosp Infect 2012; 81: 143-8.