Selective media for the quantitation of bacteria in cystic fibrosis sputum

Abstract

We used selective media together with aerobic and anaerobic incubation for the quantitation of common pathogens in liquefied sputum from children with cystic fibrosis. The accuracy of the technique was verified by reconstruction studies in which laboratory strains with antibiotic-resistance markers were added to sputum from cystic fibrosis patients. Comparison of the numbers of bacteria found on quantitative culture of clinical specimens with the 'predominant' organism found on routine culture yielded a poor correlation. When Pseudomonas aeruginosa was the most prevalent on routine culture, it was present in the highest numbers of quantitative culture (mean count = 108 cfu/g), Staphylococcus aureus (mean count = 2 x 106 cfu/g), and streptococci (mean count = 2 x 106 cfu/g) were also present in these cultures. When S. aureus was the predominant organism, H. influenzae and P. aeruginosa were also present in similar numbers (c. 107 cfu/g). When H. influenzae was the predominant species of routine culture, the mean count was 7 x 106 cfu/g and P. aeruginosa was often completely absent. We conclude that the selective technique permits reliable enumeration of sputum bacteria, and offers a more accurate assessment of the microbial flora of sputum in cystic fibrosis than does simple plating of unhomogenised sputum.