Community-acquired pneumonia in influenza season: There's a bad strain on the rise

Two articles in the European Respiratory Journal's November issue are dedicated to this dangerous and versatile microorganism.

For many years, MRSA infections were associated with hospitalised, high-risk patients.

Not only has the infection pattern recently shifted from hospital-acquired (nosocomial) to community-onset infections, but new strains have emerged, affecting young individuals, who don't have any risk factors. It seems that everything we know about these bacteria is becoming obsolete, to such an extent that a group of Italian researchers are now asking: “What is MRSA?”

This question (and the answer) is dealt with by Dr. Annalisa Pantosi from the Istituto Superiore di Sanita in Rome, Italy, and her colleagues in a current ERJ article.

Besides the hospital setting, every doctor associates MRSA with being a multiresistant bacterial pathogen – after all, even its name is based on its antibiotic sensitivity (or lack thereof). Able to destroy penicillin by producing an enzyme called penicillinase, MRSA earned its name because of a sophisticated genetic adaptation that enables it to bypass the effects of Methicillin, which blocks construction and maintenance of the bacterial cell wall. As a consequence, MRSA is resistant not only to methicillin but also to all beta-lactam antibiotics, including cephalosporins and carbapanems.

Staphylococcal infections are a concern not only from the respiratory point of view – the organ system commonly involved is the skin, with furunculosis and impetigo being frequent manifestations. In addition, they can result in serious bloodstream infections (sepsis) and can infect prosthetic implants.

The “new generation” of MRSA strains commonly causes skin and soft- tissue infections. However, even if cases of community-acquired pneumonia due to MRSA are still rare, when respiratory infections occur, they are severe and carry a high mortality.

“The estimated incidence is 0.51-0.64 cases per 100.000 people” report Dr. Konstantinos Vardakas and his colleagues from the Alfa Institute of Biomedical Sciences in Athens, Greece.

In Europe, the prevalence of infections due to this community-acquired MRSA (CA-MRSA) strain is lower than in the US. Nevertheless, physicians need to be aware of any infection that might be caused by a potentially multiple-drug resistant strain.

Moreover, MRSA-pneumonia in the community setting is often preceded by influenza or an influenza-like illness, a phenomenon that may well become relevant in the upcoming influenza season and amid the ongoing pandemic of H1N1 influenza, writes Dr. Alexandra Nakou in an accompanying editorial.

Dr. Nakou adds: “Although community-acquired MRSA is sensitive to several antibiotics such as clindamycin, trimethoprim-sulphamethoxazole and tetracyclines, the optimal treatment is not yet established. The current guidelines recommend the use of vancomycin or linezolid.”

“MRSA should be part of the differential diagnosis of severe pneumonia during the influenza season, especially in patients with evidence of necrosis and in those with a history of MRSA infection”.