Pseudomonas

It is a leading nosocomial pathogen with a high rate of mortality in sepsis. It shows resistance to many antibiotics and is very difficult to eradicate from the environment.

Human pathology

P.aeruginosa produces enzymes, exotoxin and endotoxin apart from a bluish green pigment called pyocyanin which act as virulence factors. It produces abundant glycocalyx (biofilms) or slime layer which favor its persistence in the body and outside. Such biofilms are especially seen in the lungs of patients with cystic fibrosis, and when cultured, these strains will produce mucoid colonies.

It may cause wound infections, especially in burn wounds, ventilator associated pneumonia, UTIs, malignant otitis externa, hot tub folliculitis and osteochondritis following puncture wounds in the foot. Rarely, patients suffering from Pseudomonas bacteraemia can develop a condition called ecthyma gangrenosum where the skin shows hemorrhagic bullae which ulcerate to form black eschars.

Laboratory diagnosis of Pseudomonas infections

Any Gram negative bacillus which is oxidase positive should be considered as Pseudomonas first, especially in the clinical scenarios that we discussed above. Other clues are blue green pigment on culture, fruity odor and non lactose fermenting colonies on Mac Conkey’s agar. Bacteriophage typing or pyocin typing can be done in outbreaks to locate the source and to take preventive measures.

Pseudomonas aeruginosa is a nonfastidious, Gram-negative (Fig. J), oxidase positive (Fig. H), nonfermenting rod. P. aeruginosa produces pyoverdin, a water-soluble yellow-green pigment (Fig. A, C, D1, D2). Many P. aeruginosa strains also produce the blue pigment pyocyanin (Fig. B, F). When pyoverdin combines with pyocyanin, the bright green color characteristic of P. aeruginosa is created (Fig. C). Some strains rarely produce other pigments: brown pyomelanin (D3) or red pyorubrin (D4). For the isolation and presumptive identification from clinical and environmental samples a selective agar containing cetrimide can be used (Fig. G). Fig. A, B Trypticase soy agar (B: P.aeruginosa + S.aureus), Fig. C, D, I Mueller-Hinton agar Fig. E Larger colonies of P. aeruginosa and smaller colonies of Enterococcus faecalis on tripticase soy agar. Reflected light. Fig. F Bluish colonies of P. aeruginosa, yellow colonies of S. aureus and white colonies of Enterococcus faecalis on trypticase soy agar. Fig. I Plaques are frequently found in freshly isolated strains, especially in the area of confluent growth. In some strains these are caused by phage, in others by bacteriocins.