Bactericidal activity of human serum against strains of Klebsiella from different sources.

One hundred and eighty strains of Klebsiella were tested for sensitivity to pooled normal human serum, capsular type and faecal coliform reaction. Strains of K. pneumoniae isolated from clinical infections were more likely to be serum resistant than those from the gut flora or environmental sources. For K. oxytoca strains, there was no significant correlation between serum sensitivity and source of isolation. Of 60 strains of K. oxytoca tested, 19 (32%) were serum resistant, as were 27 (23%) of 120 strains of K. pneumoniae. Strains of K. pneumoniae from human sources gave positive reactions in the faecal coliform test more frequently than strains from environmental sources. There was no correlation between a positive faecal coliform reaction and resistance to the bactericidal effect of human serum. Strains of capsular type K21 were isolated more frequently from clinical infections and were more often serum resistant than other strains. Serum resistance appears to be a virulence factor in strains of K. pneumoniae but does not account for the difference in pathogenicity between K. pneumoniae and K. oxytoca.