Pseudomonads and other nonfermenting bacilli
The nonfermentating gram-negative rods are a heterogeneous group. Most species are strict aerobes and a few can ferment compounds other than sugars. These organisms are abundant in natural reservoirs such as soil, water, and normal flora of humans. Some can cause serious infections in immunocompromised hosts. The most important are members of the genus Pseudomonas, followed by Acinetobacter, Moraxella, Alcaligenes, Flavobacterium, and Achromobacterium. All these may be acquired nosocomially so that rapid identification and epidemiological characterisation are important in tracking hospital outbreaks.
Pseudomonas
Gram-negative rods, motile
have polar rather peritrichous flagella
oxidase positive, obligate aerobes
capable of growing on simple laboratory media.
P. aeruginosa can be found in most moist environments and occasionally in the normal intestinal or skin flora. Sinks, respiratory equipment, and humidifiers are important sources of this organism in hospitals. P. Fluorescens and P. Putida are found predominantly in soil and water but can be isolated from hospital environments and contaminated food.
Pathogenecity
P. aeruginosa rarely causes disease in healthy individuals. It is a serious pathogen for patients with immunocompromised or chronically debilitated patients. Patients with cystic fibrosis, haematological malignancies or neutropenia are at risk of severe bacteraemic pneumonia. Septicaemia may result from long term IV or urinary catheterisation, surgical procedures, or severe burns. Pseudomonas endocarditis is associated with intravenous drug abuse as well as open-heart surgery. Ocular infections have resulted from contaminated opthalmological solutions. Pseudomonas osteochrondritis may result from severe trauma. P. Fluorescens and P. Putida may cause opportunistic infections in immunocompromised hosts.
Treatment
Most strains of P. Aeruginosa are resistant to relatively high levels of most antibiotics in use. Acute life-threatening infections are treated by a combination of tobramycin and an anti-pseudomonal b-lactam. Unlike P. Aeroginosa, P. Fluorescens and P. Putida are sensitive to a wide range of antibiotics.
The pseudomallei group include several opportunistic pathogens: P mallei, P. Pseudomallei. P cepacia, and P. picketti. P. mallei is the only non-motile member of the genus Pseudomonas and is the causative agent of glanders, a severe infection of horses that is occasionally transmitted to animal handlers. The disease is characterised by pneumonia and by necrosis of muccous membranes, skin, and lymphatics.
P. pseudomallei is found in the soil and water in tropical areas. It is the causative agent of melioidosis, a disease which is prevalent in the tropical parts of Asia. The organism is usually inhaled and may remain dormant for several months or even years. Chronic pneumonia, resembling tuberculosis, may develop. The organism can also cause an acute suppurative infection in traumatised skin lesions and can progress to septicaemia. Patients usually respond to chloramphenicol, tetracycline, and trimethroprim-sulphonimides.
P. cepacia has long been recognised as a pathogen for plants. In recent years, it had been associated with a number of hospital outbreaks. It can cause the same range of disease as P. aeruginosa, including pulmonary disease in cystic fibrosis patients. It is also resistant to a number of antimicrobial agents. Infections can usually be treated by chloramphenicol or trimethorim-sulphonimides and with some cephalosporins.
The genus acinetonacter consists of a single species A. calcoacetius. There are 2 variants: A. calcoacetius var anitratus oxidises glucose with production of acid, whereas var. lwoffi lacks this trait. These organisms are short gram-negative rods and are frequently found in pairs. They are non-motile (hence the name), grow only aerobically and are oxidase-negative. Acinetobacter can be found widely distributed in the environment and in the normal flora in approximately 10% of individuals. Nosocomial outbreaks of acinetobacter pneumonia and bronchitis have been reported. Intravenous catheterisation sometimes lead to septicaemia. Infections are usually treated with carbenicillin, co-trimoxazole, or kanamycin.
The flavobacterium are non-motile, slender gram-negative rods that form yellow colonies on most solid media. Flavobacteria are widely distributed in nature. F. meningosepticum can be highly virulent for the newborn infant, especially the premature. It has caused outbreaks of septicaemia and meningitis with a high mortality rate. These infections are usually attributed to contaminated hospital equipment and solutions. However, the organism has also been isolated from the female genital tract. Flavobacterium also causes bacteraemia in postoperative patients, although the illness is much milder. The organism has an unusual antibiotic sensitivity pattern for a gram-negative bacillus; it is resistant to aminoglycosides and penicillins but susceptible to erythromycin, rifampin and vancomycin.
Moraxella
Members of the genus moraxella are similar to acinobacter but are oxidase-positive and highly sensitive to penicillin. They are easily confused with Neisseria. Moraxella lacunata is a rare cause of conjunctivitis and corneal infections. M. osloensis, nonliquefaciens, and phenylpyruvica are members of the normal flora but may be involved in serious infections.
Achrombacter
Achromobacter xylosoxidans is a motile oxidase-positive organism first described in 1971. It occurs in lower animals as well as free-living in nature. It has been associated with a variety of human diseases including meningitis, septicaemia, and otitis media.
Alcaligenes
Alcaligenes faeclis is oxidase-positive and usually motile. It may be encountered in the faeces or sputum as a harmless saprophyte but it has been associated with serious infections. As a contaminant of irrigation fluids and intravenous solutions, it has caused epidemics of urinary tract infections and postoperative septicaemia.