Production of nonmucoid Pseudomonas aeruginosa alginate grown anaerobically in vitro that does not change its phenotype. On Pseudomonas isolation agar, P. aeruginosa strain PAO1 revealed a nonmucoid phenotype when cultivated aerobically (A) or anaerobically (C), whereas its isogenic mutant PDO300 showed only a mucoid phenotype (B). When indirect immunofluorescence was used, alginate production of PAO1 was detectable under anaerobic growth conditions (F) but not under aerobic growth conditions (D), whereas PDO300 was alginate-positive under aerobic growth conditions (E). Bars in D–F, 1 μm.