Pseudomonas aeruginosa is a common Gram-negative, rod-shaped bacterium that can cause disease in plants and animals, including humans. A species of considerable medical importance, P. aeruginosa is a prototypical "multidrug resistant (MDR) pathogen" recognised for its ubiquity, its intrinsically advanced antibiotic resistance mechanisms, and its association with serious illnesses – especially nosocomial infections such as ventilator-associated pneumonia and various sepsis syndromes. The organism is considered opportunistic insofar as serious infection is often superimposed upon acute or chronic morbidity – most notably cystic fibrosis and traumatic burns – or found in immunocompromised individuals, but the organism does produce a range of clinically important infections in the immunocompetent and/or in situations where no pre-existing vulnerability is required e.g. hot tub folliculitis. In all infections produced by P. aeruginosa, treatment is dually complicated by the organism's resistance profile, which may lead to treatment failure and/or expose patients to untoward adverse effects from advanced antibiotic drug regimens. This dilemma is a central clinical problem in the field of antimicrobial resistance. It is citrate, catalase, and oxidase positive. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, thus has colonized many natural and artificial environments. It uses a wide range of organic material for food; in animals, its versatility enables the organism to infect damaged tissues or those with reduced immunity. The symptoms of such infections are generalized inflammation and sepsis. If such colonizations occur in critical body organs, such as the lungs, the urinary tract, and kidneys, the results can be fatal. Because it thrives on moist surfaces, this bacterium is also found on and in medical equipment, including catheters, causing cross-infections in hospitals and clinics. It is implicated in hot-tub rash. It is also able to decompose hydrocarbons and has been used to break down tarballs and oil from oil spills. P. aeruginosa is not extremely virulent in comparison with other major pathogenic bacterial species – for example Staphylococcus aureus and Streptococcus pyogenes – though P. aeruginosa is capable of extensive colonization, and can aggregate into enduring biofilms. P. aeruginosa does not fare especially well under suboptimal atmospheric conditions. P. aeruginosa is among the Gram-negative bacilli commonly isolated from the exoskeletons and/or droppings of the cosmopolitan peridomestic cockroaches – including Periplaneta americana and Blatella germanica – which are often pervasive in households, as well as hospital settings. The importance of P. americana (and other vermin) as a potential reservoir or vector of P. aeruginosa continues to be studied.

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