The threat of persistent bacteria and fungi contamination in tuberculosis sputum cultures

Abstract

Background: Tuberculosis (TB) sputum culture contaminants make it difficult to obtain pure TB isolates.We aimed to study and identify persistent TB sputum culture contaminants post the standard laboratory pre-culture sample decontamination techniques. Methods: This was a longitudinal study of TB sputum culture contamination for a cohort of TB patients on standard treatment at: baseline, during TB treatment and post TB treatment. Sputum samples were decontaminated with 1.5%NaOH and neutralized using 6.8 Phosphate buffer solution.Sputum was then inoculated into MGIT (mycobactrial growth indicator tube) supplemented with 0.8ml PANTA. A drop of each positive MGIT culture was sub cultured onto blood agar and incu- bated for 48 hours at 35 -37OC.Any growth was identified using growth characteristics and colony morphology. Results: From October 2017 through May 2019;we collected 8645 sputum samples of which 8624(99.8%) were eligible and inoculated into MGIT where 2444(28.3%)samples were TB culture positive and 255(10.4%)were positive for contam- inants:237 none-tuberculosis bacteria, 12 fungi and 6 mixed(none-tuberculous bacteria+fungi).There was no statistically significant difference between none tuberculosis bacteria and fungi in the treatment (OR=1.4,95%CI:0.26–7.47,p=0.690) and the post treatment TB phases(OR=2.02,95%CI:0.38–10.79,p=0.411)Vs baseline. Conclusion: None-tuberculous bacteria and fungi dominate the plethora of TB sputum culture contamination and persist beyond the standard laboratory pre-culture decontamination algorithm. Keywords: Bacteria; Fungi; Inoculation; PANTA (Polymyxin B; Amphotericin B; Nalidixic acid; Trimethoprim; Azlocillin).