Paraffin embedded tissues: sterile or contaminated material?
Authors: Barros Mirna Duarte , Liquidato Bianca Maria , Adamescu Aida Isabela , Navarini Alessandra Isabela , Mimica Marcelo Jenne 
Published: November 2023

Introduction

Formalin‐fixed and paraffin‐embedded tissue is an invaluable resource in research. Formaldehyde fixation has been the method of choice for tissue preservation in pathology for more than a century. Beside tissue structure, the cellular contents of proteins and nucleic acids are preserved by the cross‐linking between and within proteins in the tissue. This gives a tissue sample resilience to long‐term storage, usually at ambient room temperature. When any kind of tissue is removed, it is sent to a pathology lab for processing, examination, and diagnosis. This material is processed by a very well known technique, that comprises inclusion in paraffin. Such method allows preparation of slides to undergo histological analysis. After diagnosis is done, the remaining paraffin blocks are stored almost indefinitely.

Objectives

This paper aims to investigate whether metabolically active bacteria could be detected within tonsils and lymphnodes removed for different reasons, and stored after proper histological examination in paraffin blocks. Moreover, we wanted to know if there would still be bacteria growing in the preserved tissues, as remaining microbiote or new bacterial contamination. At last, if these bacteria are present, could it affect the tissue structure.

Material and methods

Twenty three paraffin blocks containing tissues from different lymphatic organs were sellected: nasopharyngeal tonsils, palatine tonsils and lymph nodes. The hole blocks were imprinted into agar‐cellulose media, and thin slices of the blocks were made and put in liquid culture tubes. The plates and culture tubes were then incubated overnight at an incubation temperature of 37 °C (98.6 °F). The blocks were then cleaned of surface paraffin and then the tissue inside was carved to obtain material for slides. Slides containing samples of each one of the specimens were submitted to Zieh‐Neelsen coloring, in order to investigate the presence of acid‐alcohol resistant bacilli (AARB).

Results

Two palatine tonsil imprints were positive for Negative‐coagulase Staphylococcus, the same happened with 5 Nasopharyngeal tonsil imprints and 2 Lymph node imprints. All culture plates showed only one bacterial colony localized on the border of the imprinted block. All liquid culture tubes were negative for bacterial growth. All of the slides submitted to Zieh‐Neelsen coloring were negative to (AARB).

Discussion

Negative‐coagulase Staphylococci may be present on the skin of the hands. Tissue embedded paraffin blocks are routinely and freely handled without use of latex gloves, so we can assume that probably, S. epidermidis was the colonizer, and it is possible to infer that it was caused by handling contamination of the surface of the blocks. Paraffin embedded blocks seem to remain a safe and easy way for tissue sample storage, specially over time, that ranged from 3 to 15 years in this study.

Conclusions

Only handling bacterial contamination could be detected within tonsils and lymph nodes stored in paraffin blocks. Paraffin embedded blocks seem to remain a safe and easy way for tissue sample storage.