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P. aeruginosa is typically described as a 'rod-shaped' bacteria. The second abstract below speaks of its two mutant strains, PAO1 and PBP2, that are sphere-shaped.


Pseudomonas

"P. aeruginosa isolates may produce three colony types. Natural isolates from soil or water typically produce a small, rough colony. Clinical samples, in general, yield one or another of two smooth colony types. One type has a fried-egg appearance which is large, smooth, with flat edges and an elevated appearance. Another type, frequently obtained from respiratory and urinary tract secretions, has a mucoid appearance, which is attributed to the production of alginate slime. The smooth and mucoid colonies are presumed to play a role in colonization and virulence."

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Function of penicillin-binding protein 2 in viability and morphology of Pseudomonas aeruginosa -- Legaree et al. 59 (3): 411 -- Journal of Antimicrobial Chemotherapy

"Results: PAO1 grew with a spherical morphology in the presence of mecillinam at concentrations as high as 2000 mg/L.

The PAO1 {Delta}pbpA insertional mutant also grew as spheres, and complementation with a plasmid encoding active pbpA, but not with an inactive Ser-327.

With P. aeruginosa at this concentration of mecillinam, cells retained their normal rod shape but, surprisingly, a morphological change to spherical cells was observed when incubated at concentrations between 200 mg/L and 400 mg/L (Figure 2).

Characterization of PBP 2

To ensure that the proteins being investigated were indeed either the full-length or truncated PBP 2 from P. aeruginosa, N-terminal sequence analysis and mass-spectrometry fingerprinting were performed. The N-terminal amino acid sequences obtained corresponded to those expected for each protein (Figure 1), and the MS-Fit42 gave a MOWSE score of 3.3 x 107, thus confirming the identity of P. aeruginosa PBP 2.

PAO1 {Delta}pbpA mutants lacking a functional pbpA gene were observed to grow in LB broth at 37°C, albeit at an apparent slower rate compared with wild-type PAO1 cells (Figure 7). SEM analysis revealed that, like wild-type cells treated with mecillinam, the PAO1 {Delta}pbpA mutant grows with a spherical morphology, although perhaps with a little less uniformity in appearance (Figure 7). Normal rod-shaped morphology and cell dimensions were restored upon transformation of the mutant with the complementation vector pACBL31 which contained a functional copy of pbpA."