Pseudomonas bacteria (Pseudomonas aeruginosa) colonies being cultured on an agar growth medium (blue) in a petri dish. P. aeruginosa is a non- invasive and opportunistic pathogen that rarely infects healthy individuals. It infects patients with compromised immune systems, and is a major cause of hospital-acquired (nosocomal) infections. Grown on tryptic soy agar (TSA), a nutrient agar.