Laboratory Medicine and Nosocomial Infections: the Usual Suspects

The most common microorganisms causing nosocomial infections are bacteria, including commensal bacteria, which are part of the normal flora, and pathogenic bacteria, which come from an exogenous source. Commensal bacteria normally protect against pathogenic bacteria but can cause infection in the compromised host. Pathogenic bacteria are more virulent and cause infections regardless of host status.

Viral infections are actually the most common etiologies of nosocomial infections, causing many viral respiratory infections and rotaviral infections in winter, and many enteroviral infections in the summer. Hepatitis B virus (HBV), Hepatitis C virus (HVC), respiratory synctial virus (RSV), rotaviruses, and enteroviruses are transmitted nosocomially. When there has been long antibiotic treatment and/or severe immunosuppression, such as occurs with transplant and oncology patients, fungi such as Candida albicans, Cryptococcus neoformans, and Aspergillus spp., as well as parasitic protozoa and other opportunistic organisms like Cryptosporidium can also cause infections.

Mycoplasmas such as Mycoplasmas pneumoniae and M. homini have also been diagnosed in patients as causes of nosocomial infections.

Ectoparasites including scabies, mites, maggots, and fleas, can be acquired in hospitals. In immunosuppressed patients, cases of crusted scabies, where there is hyperkeratosis of the skin, literally thousands of female mites can be present and shed into the environment. They are then transmitted to the hospital staff and to other patients. When pets are able to enter a hospital or a research institute attached to a hospital, workers, but not necessarily patients can become infested by ectoparasites such as fleas.

Maggots become a problem when wound dressings are not changed frequently enough. The odor emanating from the wound can attract flies, which lay eggs on the dressing. After approximately 12 hours, the eggs hatch to larvae, which can pass though the dressing. In addition, free-living insects such as ants and cockroaches, which are not parasites, might act as vectors or transmitters of infections.