Pseudomonas aeruginosa is a gram negative bacillus which is notorious for causing nosocomial, or hospital-borne infections. It belongs to the family Pseudomonadaceae, is actively motile utilizing polar flagella and has minimal nutritional requirements for survival. It is a non-fermentor of glucose and can only utilize glucose via oxidation in an aerobic environment. The majority of the clinical isolates of Pseudomonas aeruginosa produce two soluble pigments, a fluorescent green pigment, pyoverdin, and a blue pigment, pyocyanin. Certain strains also produce the red pigment called pyorubin. Additionally, the organism is characterized by a signature fruity odor.
In 1862, Luke documented the presence of microscopic, rod shaped bacilli from blue green pus. The organism was isolated by Gessard in 1882 and named Bacillus pyocyaneus. Over the years, Pseudomonas aeruginosa has emerged as a major human pathogen due to its capacity to cause infections in a variety of patients with compromised immunity related to such things as burns, wound infections and respiratory infections. Furthermore, the emergence of multi-drug resistant strains in hospital settings resulting in wound, lung and urinary tract infections has increased alarmingly. The treatment of such infections is complex and has a high risk of mortality.
The organism is capable of surviving temperatures of 45° C to 50° C. It can also grow in distilled water utilizing carbon dioxide, residual sulphur, phosphorus, iron, divalent cations, etc. as the source of carbon and essential nutritional substrates. It cannot, however, survive at a pH of 4.5 or lower.
Pseudomonas aeruginosa is prevalent in areas where a moist environment persists. In the hospital environment, it is readily accessible to water in sinks and drains, toilets and flowerpots. The prevalence of Pseudomonas aeruginosa in hospitals of US is about 0.4 % and it is the fourth most common isolate as a nosocomial pathogen. It accounts for 10.1% of all hospital acquired infections. The community at large is also at risk as infections are acquired in areas that are associated with a moist environment such as swimming pools, common baths, hot tubs etc.
Pseudomonas is an opportunistic pathogen. It frequently attacks patients whose immune system is suppressed such as diabetics, patients on chemotherapy, patients who have undergone a major surgery or those who have invasive devices in place for more than one week. Infections due to Pseudomonas aeruginosa are both toxigenic and invasive.
The pathogenesis of pseudomonal infections are multifactorial. According to Pollack the stages that are important in the pathogenesis of pseudomonal infections are:
Colonization is an important preceding event in patients with cystic fibrosis, ventilator associated pneumonias or nosocomial urinary tract infections. It has been observed that the gastrointestinal tract of about 20% of patients admitted for hospitalization of more than 72 hours are eventually colonized by Pseudomonas aeruginosa. Enzymes such as proteases secreted by the organism further assist in the destruction of tissues and spread the infection.
Infections caused by Pseudomonas aeruginosa
The common infections caused by Pseudomonas aeruginosa are as follows:
Respiratory infections: Recurrent infections with P.aeruginosa are common in patients affected with cystic fibrosis. Morphologically, these strains are typically mucoid when cultured on artificial media. Pseudomonas aeruginosa is one of the leading causes of ventilator associated pneumonias in intensive care settings. Pneumonia following systemic infection is common among patients on chemotherapy and patients of Acquired Immunodeficiency Syndrome.
Bacteremia & endocarditis: Pseudomonas aeruginosa bacteremia colonization is frequently noted following use of invasive devices. Mortality may be as high as 10% in such cases. Endocarditis is a notable finding in intravenous drug abusers and has presents with a very high mortality rate.
Central Nervous system: Meningeal infections usually occur as a result of contiguous infections from the paranasal sinuses, ear involvement or invasive procedures.
Ear infections: Pseudomonas aeruginosa is the most common causative organism of otitis externa (swimmer’s itch). In patients with chronic otitis media, pseudomonas is a significant pathogen due to its potential to destroy tissues and spread towards deeper structures. Sometimes this may result in facial nerve paralysis.
Eye: Pseudomonas aeruginosa has been known to cause the loss of an eye. The primary source of Pseudomonas aeruginosa endophthalmitis is inappropriate aseptic technique during surgery or the use of contaminated irrigating solutions.
Bone, joint and soft tissue infections: Bone and joint infections due to Pseudomonas aeruginosa have amplified in recent years. The primary cause is related to a large number of replacement surgeries that are being performed for chronic conditions such as rheumatoid arthritis or osteoarthritis. Soft tissue and spreading infections are frequently seen in patients suffering from foot disease related to diabetes. Typically, the wound presents as a nonhealing ulcer with fruity smelling green slough.
Skin infections: Pseudomonas aeruginosa survives on moist skin and therefore is implicated in nail fold infections known as the Green Nail Syndrome. Ecthyma gangrenosum is a distinct entity that is classically associated with Pseudomonas aeruginosa infections. It is habitually associated with pseudomonal bacteremias. Pseudomonas aeruginosa is also the most essential causative organism for burn wound infections.
Mechanism of drug resistance in Pseudomonas aeruginosa
Pseudomonas aeruginosa is capable of producing biofilms. These biofilms drastically reduce the accessibility of various drugs to the bacterial cells within the them and therefore result in complexity in the treatment of infections.
Pseudomonas aeruginosa is known to produce extended spectrum betalactamases (ESBL) which are capable of hydrolyzing the third generation cephalosporins which are frequently used as the first line of therapy in many clinical settings. Such strains are likely to be susceptible to carbapenams such as imipenam and meropenem.
Metallobetalactamases (MBL): Pseudomonas aeruginosa has acquired yet another mechanism of drug resistance in which it is capable of hydrolyzing carbapenams. Carbapenams are currently the highest group of antimicrobial agents available in severe cases of sepsis or infections. Such strains are often Multidrug resistant (MDR) leaving only one available choice of therapy which is colistin.
Resistance to chemical disinfectants
Pseudomonas aeruginosa is notorious for its innate resistance to chemical disinfectants, a property conferred by its outer membrane. The high Mg2+ content of the outer membrane is responsible for strong lipopolysaccharide linkages which do not permit the penetration of the disinfectant molecules. Resistance to the commonly used antiseptics like quaternary ammonium compounds, hexachlorophene, cetrimide etc. has been noted. The organism is susceptible, however, to acids, gluteraldehyde, silver salts and strong phenolic disinfectants.