The genus Pseudomonas consists of a number of human pathogens, the most important of which is Pseudomonas aeruginosa. P aeruginosa is an opportunistic pathogen found widely in soil, water, and organic material, reflecting its limited nutritional requirements. A moist environment is favored. Human colonization in the community is rare, and, when it occurs, the skin, gut, and upper or lower airway are colonized.
This organism is endemic in Southeast Asia with the highest prevalence in Thailand. The organism is a saprophyte living in the soil. Infection may be subclinical, acute, subacute, or chronic.
Paeruginosa infections may occur in patients with AIDS. Risk factors for infection include a CD4 count of < 100 cells/mL3, neutropenia or functional neutrophil defects, intravascular catheterization, hospitalization, and prior use of antibiotics including ciprofloxacin or trimethoprim-sulfamethoxazole. Many cases are community acquired. Bacteremia is common, and the lung or an intravenous catheter is the most frequent portal of entry.
P aeruginosa endocarditis occurs predominantly in two settings: in association with IV drug use (IVDU) or with prosthetic valve endocarditis (PVE). The majority of cases of native valve endocarditis caused by P aeruginosa occur in association with IVDU. Risk factors for endocarditis associated with IVDU include the use of substances that are not boiled after mixing before injection, the injection of drugs at shooting galleries, and the use of pentazocine and tripelennamine.
Bone and joint infections caused by P aeruginosa may result as complications of surgery, in particular the implantation of joint prostheses, or pelvic or genitourinary surgery, in association with intravenous (IV) drug abuse, trauma resulting in open fractures such as motor vehicle or farm related accidents, complicated UTIs, diabetic foot ulcers, or puncture wounds of the foot. P aeruginosa has a predilection to infect fibrocartilaginous structures.
P aeruginosa infection of the external auditory canal may be acute or may be a chronic serious infection called malignant otitis externa. Acute diffuse otitis externa is often referred to as swimmer’s ear. P aeruginosa may be part of the normal flora of the external auditory canal or may colonize the canal as a result of exposure to water.
Infections caused by P aeruginosa involving the skin may be primary or secondary. Secondary infections have been described above and include ecthyma gangrenosum, subcutaneous nodules, vesicles, bullae, cellulitis, deep abscesses, and necrotizing fasciitis. Primary skin lesions are noted as complications of neutropenia, burns, decubitus ulcers, prematurity, exposure to a moist environment, and hydrotherapy. Burn wound sepsis is a serious complication that may be caused by P aeruginosa.
Patients with CF demonstrate particularly complex host-parasite interactions involving P aeruginosa. CF is characterized by mutations in the CF transmembrane conductance regulator resulting in abnormal chloride ion secretion and cellular dehydration. A continuous cycle of cellular inflammation with increased numbers of neutrophils in bronchoalveolar lavage fluid, increased interleukin 8 secretion, viscous mucus, mucus plugging of airways, and infection results.
Pseudomonas lung infections occur in patients with chronic lung disease or impaired immunity, usually in association with nosocomial factors such as endotracheal intubation, respiratory therapy, prolonged hospitalization, antibiotic use, and neutropenia. Pneumonia takes two forms: primary and bacteremic. Primary pneumonia arises in predisposed patients following nosocomial colonization and aspiration of P aeruginosa. Pneumonia is characterized by fever, tachypnea, cough with purulent sputum, shortness of breath, cyanosis, and often signs of sepsis.
The Enterobacteriaceae are a diverse family of bacteria that, in nature, exist in soil, on plant material, and in the intestines of humans and other animals. Another ecological niche in which these organisms thrive is the hospital. Many of these organisms cause a wide variety of extraintestinal diseases that are often nosocomial and commonly present in debilitated or immunocompromised hosts.