LEGIONELLA

(under the microscope)
The first discovery of bacteria from genus Legionella came in 1976 when an outbreak of pneumonia at an American Legion convention led to 29 deaths. The causative agent, what would come to be known as Legionella pneumophila, was isolated and given its own genus. The organisms classified in this genus are Gram-negative bacteria that are considered intracellular parasites. They grow well on buffered charcoal yeast agar, but it takes about five days to get sufficient growth. When grown on this medium, Legionella colonies appear off-white in color and circular in shape. Laboratory identification can also include microscopic examination in conjunction with a direct flourescent antibody (DFA) test. Direct specimens of Legionella do not stain well with a Gram-stain.

Since the initial discovery many species have been added to the Legionella genus, but only two are within the scope of our discussion: L. pneumophila and L. micdadei.

Motile
Urease -
Catalase +
Nitrate -
Gelatinase +

L. pneumophila

L. pneumophila is the bacterium associated with Legionnaires' disease and Pontiac fever. Respiratory transmission of this organism can lead to infection, which is usually characterized by a gradual onset of flu-like symptoms. Patients may experience fever, chills, and a dry cough as part of the early symptoms. Patients can develop severe pneumonia which is not responsive to penicillins or aminoglycosides. Legionnaires' disease also has the potential to spread into other organ-systems of the body such as the gastrointestinal tract and the central nervous system. Accordingly, patients with advanced infections may experience diarrhea, nausea, disorientation, and confusion. The 1200 or so cases of Legionnaires' disease per year in the United States usually involve middle-aged or immunosuppressed individuals. Pontiac fever is also caused by L. pneumophila but does not produce the severity of the symptoms found in Legionnaires' disease. The flu-like symptoms are still seen in Pontiac fever patients but pneumonia does not develop and infection does not spread beyond the lungs. Most L. pneomophila infections are easily treated with erythromycin.

Beta-lactamase +
Hippurate hydrolysis +

L. micdadei

L. micdadei is the second most commonly isolated member of Legionella. This bacterium can cause the same flu-like symptoms and pneomonia which characterize an L. pneumophila infection. Unlike its relative, L. micdadei is sensitive to the penicillins because it does not produce beta-lactamase.

Beta-lactamase -
Hippurate hydrolysis -
Acid fast