Pseudomonas aeruginosa strains from nosocomial pneumonia are more serum resistant than P. aeruginosa strains from noninfectious respiratory colonization processes.

Background: Serum resistance is regarded as a major virulence factor of bacteria and is thought to be mediated by 0 side chains of the lipopolysaccharides (LPS). We investigated the serum-resistance properties and 0 serogroups of Pseudomonas aeruginosa strains isolated from intensive careunit (ICU) patients with pneumonia and from the respiratory tract of ICU patients without respiratory tract infections. Materials and Methods: 171 P. aeruginosa strains were consecutively isolated from bronchoalveolar lavage fluid or transtracheal aspirates of ICU patients with monobacterial nosocomial pneumonia and 49 strains were isolated from the respiratory tract of ICU patients without respiratory tract infections. All strains were 0 serogrouped using 0-antigen-specific sera for 14 0 serogroups and tested for their sensitivity to the serum's bactericidal effect. Results: Using two different analyses, the frequency of serum-sensitive isolates was significantly lower in strains from patients with pneumonia (56.1%; n = 96/171 and 22.8%, n = 39/171, respectively) than in strains from asymptomatically colonized patients (73.46%; 36/49 and 38.8%, n = 19/49, respectively) (p = 0.03; OR = 2.163; 95% CI = 1.072-4.368 and p = 0.0289; OR = 2.144; 95% CI = 1.089-4.368, respectively). 0 serogrouping revealed higher frequency of the serogroups A (11.9% and 16.3%, respectively), B (14.3% and 21%), E (26.5% and 24.6%), and I (28.6% and 28%) in both strain collections. The frequency of serum-sensitive strains (13/28 and 3/45, respectively) was significantly lower among strains expressing the A and B serogroups, than for all other serogroups (p < 0.05). Conclusion: Strains isolated from patients with pneumonia and strains possessing 0-A or 0-B serogroups appear to have greater pathogenic potential by virtue of their ability to resist serum-mediated killing. The linkage, however, between the 0 serogroups, serum resistance, and a strain's virulence remains unclear at this stage.