PP2 -Evaluation of enrichment technique to enhance culture positivity of sterile fluids

Abstract

Introduction

Infection of sterile body fluids by microorganisms can lead to severe morbidity and mortality. The low yield of pathogens from direct cultures poses a significant problem in managing these patients. The culture of specimens following enrichment helps overcome this drawback.

Objectives

To assess the isolation rates in direct and enriched cultures of sterile fluids and to identify the causative bacteria.

 Design, setting and methods

A retrospective, cross-sectional study was conducted in the Microbiology Laboratory of the District General Hospital, Matara. All laboratory records from January 2021 to October 2022 were reviewed, and sterile specimens that were simultaneously inoculated into both routine culture media and enriched media (BHI broth) were included in the study. The direct culture plates were incubated for 48 hours, whereas inoculated BHI broth was incubated for up to 5 days. In cases where no growth was observed in direct cultures, BHI broth was processed for the identification of causative microorganisms upon visual observation of turbidity. Data were analyzed using SPSS version 28.

 Results

Of 840 sterile fluid samples analyzed, 32.6% were ascitic fluid (n=274), followed by peritoneal fluid (n=178, 21.2%), pleural fluid (n=105, 12.5%), joint fluid (n=100, 11.9%), abdominal fluid (n=24, 2.9%) and other fluids (n==159, 18.9%).

A total of 210 (25%) samples yielded positive bacterial growth. Direct cultures isolated 17.7% (n= 149). Among the 691 samples that were initially negative in direct cultures, 61 (8.8%) became positive after enrichment. The direct cultures yielded coliforms (n=78, 52.4%), Pseudomonas spp (n=26, 17.4%), Staphylococcus aureus (n=23, 15.4%), Streptococcus spp (n=9, 6.0%) and Coagulase negative staphylococcus (CoNS) (n=6, 4.0%). The bacteria isolated from enriched samples were CoNS (n=17, 27.9%), coliforms (n=14, 23.0%), Pseudomonas spp. (n=11, 18.0%), Streptococcus spp. (n=10, 16.4%), and Staphylococcus aureus (n=6, 9.8%). Among positive enriched cultures, 18 (29.5%) were ascitic fluid, followed by peritoneal fluid (n=10, 16.4%), joint fluid (n=5, 8.2%), and other fluids (n=28, 45.9%).

Conclusion

Enrichment of sterile fluids improved the yield of microorganisms & enhanced the culture positivity rates, improving the detection of bacterial pathogens that might be missed in direct cultures. However, the possibility of contamination by CoNS in enriched cultures remains a limitation of this study due to the lack of clinical correlation.