Serotypes and antibiotic susceptibilities of Pseudomonas aeruginosa isolates from single sputa of cystic fibrosis patients.

A phenotypic characterization of Pseudomonas aeruginosa from single sputum samples of 21 
typical cystic fibrosis patients indicated a high frequency of heterogeneity among 
isolates on the basis of differences in antibiotic resistance, colony morphology, 
pigmentation, and serotype. Two or more isolates with different but stable 
susceptibilities to carnecillin, gentamycin, streptomycin, tetracycline, chloramphenicol, and sulfamethoxazole plus trimethoprim 
were detected in 38% of the sputa. Differences generally were independent 
of the mucoid state of the strain. O- antigen group determination 
with the Difco typing set showed that two or more serologically 
distinct strains were present in 10/21 sputum specimens. Nonmucoid derivatives of 
mucoid isolates almost always retained both the antibiotic susceptibilities and serotype 
of their parent strain. These data suggest that cystic fibrosis patients 
may be cocolonized/coinfected by difference strains of P. aeruginosa more frequently 
than generally believed. Alternatively, phenotypically distinct strains from a single patient 
might arise as phenotypic dissociants from a single infecting strain. Because 
of the frequency and multiplicity of phenotypically distinct P. aeruginosa isolates 
which we obtained from our cystic fibrosis patients, it is important 
to select multiple isolates from sputum cultures for antimicrobial susceptibility testing 
so as to assess adequately the susceptibility of this organism to 
antibiotic therapy in cystic fibrosis. We recommend that several colonies of 
each distinguishable colony type of P. aeruginosa be pooled for the 
antibiogram. 