Are CA-MRSA more virulent than HA-MRSA?

Community-associated MRSA (CA-MRSA) are characterised by the ability to cause infections – and sometimes serious invasive infections – in previously healthy individuals without healthcare contact. We don’t see this clinical manifestation in healthcare-associated MRSA (HA-MRSA), so it’s is logical to conclude that CA-MRSA are somehow more virulent than HA-MRSA. However, a recent study published in the Journal of Medical Microbiology shows that CA-MRSA strains were no more virulent than HA-MRSA strains in a battery of laboratory tests. This suggests that CA-MRSA’s ability to cause infections in healthy individuals without healthcare contact has more to do with transmissibility than virulence. And this explains the curious phenomenon that CA-MRSA seem to cause the same spectrum of disease as HA-MRSA when they infiltrate a healthcare setting.

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

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