Detection of microorganisms on formalin-fixed and stored pathology tissues: A microbiological study

Formalin is widely used to fix histological preparations and as preservatives in embalming solutions and is an age-long practice in medical laboratories. It is generally accepted that the risk of contracting infections is relatively high among medical laboratory workers and pathologists. Recent studies have, however, suggested that formalin does not effectively inactivate all kinds of microbes in formalin-fixed tissue (FFT). Long time preserved tissues in formalin may develop growth of microbes on the surface of the formalin.

The purpose of the study is to determine the growth of microorganisms on the surface of FFTs.

Fifty-one containers of 10% formalin with fixed tissues and undiscarded formalin solution not containing tissues of years 2013-2015 (17 in each year) were selected, and samples for inoculation onto the cysteine lactose electrolyte deficient agar plates were taken from the surface of the FFT using sterile cotton tips. The growth of the colonies was checked for after 48 h.

Out of 51 samples from 2013 to 2015, 17 had shown growth of microbial colonies. Six out of 17 samples of 2013, 7 out of 17 of 2014 and 4 out of 17 samples of 2015 had colonies of microbes on agar plates. Gram-negative bacilli, Bacillus subtilis and micrococci were mostly found.

There were viable microbes on the surfaces of formalin solution containing pathology tissue. Since cross-contamination by microbes may occur during regrossing or processing, protocols to decrease cross-contamination should be instituted.