MRSA and multidrug-resistant gram-negative bacilli, such as P. aeruginosa and extended-spectrum beta-lactamase–producing gram-negative bacilli, should be considered in patients who have certain comorbidities, who have recently received antibiotics, and/or who have had exposure to healthcare settings.
Elicit history of specific exposures (eg, Histoplasma spp and bat or bird droppings, C. psittaci and birds).
Anaerobes should be suspected on clinical grounds when there is aspiration, a pulmonary cavity in an aspiration-prone patient, putrid discharge (sputum or empyema fluid), and/or no likely aerobic pathogen. Putrid sputum or empyema fluid is considered diagnostic of infection with anaerobic bacteria, primarily Bacteroides melaninogenicus, Peptostreptococcus, and Fusobacterium nucleatum.
Legionella accounts for 1 to 10 percent of cases of CAP. Legionella can occur as a sporadic infection or cause outbreaks. Travel-associated legionellosis is becoming more common. In most instances, Legionella is transmitted to humans by inhalation of aerosols containing the bacteria. Outbreaks have been associated with exposure to a variety of aerosol-producing devices, including showers, a grocery store mist machine, cooling towers of air conditioning systems, whirlpool spas, and fountains
Environmental exposures — Most Legionella infections are associated with exposure to contaminated manmade water reservoirs. Reported sources of infection are diverse and include showers [57], pools [58], hot tubs [59], aquariums [44], fountains [60], birthing pools [61,62], drinking water systems [14,51], air conditioning systems and cooling towers [63,64], and rainwater collection systems [65]. Natural water systems, such as rivers and streams, are less common sources of infection [66].
Soil, potting mix, and compost exposure is also a risk factor for Legionella infection, particularly for L. longbeachae [7,67]. Gardening and specifically not washing one's hands after gardening are associated with an increased likelihood of infection with L. longbeachae [7,67].