# Evaluating bacterial growth in raw, frozen, and heat-treated colostrum inoculated with fecal Escherichia coli

## TL;DR

This study found that raw and frozen colostrum inhibit E. coli growth, but heat treatment reduces this antibacterial effect, affecting colostrum management practices.

## Contribution

The study reveals that heat treatment diminishes the natural antibacterial properties of bovine colostrum compared to raw or frozen storage.

## Key findings

- Raw and freeze-thawed colostrum inhibited E. coli growth.
- Heat-treated colostrum lost its antibacterial activity.
- Colostrum replacer did not inhibit E. coli growth.

## Abstract

Summary: This project evaluated the effect of postharvest thermal processing on the growth-promoting or -inhibiting activity of bovine colostrum. Colostrum was collected aseptically from Holstein cattle (n = 7), and aliquots were submitted to 3 treatments: cooling at 4°C only (RW), freezing at -20°C (FR), or heat treatment at 60°C for 60 minutes (HT). Colostrum replacer (CR) was included as a control. The day after treatments were applied, each sample was inoculated with a bovine gastrointestinal strain of Escherichia coli and incubated together at 37°C for 24 hours. Growth of E. coli was quantified on MacConkey agar plates immediately after inoculation (0 hours) as well as at 2, 4, 6, 8, and 24 hours. The resulting growth curves showed that cooled and freeze-thawed colostrum inhibited growth of E. coli, whereas heat-treated colostrum and colostrum replacer did not. These findings show that HT reduced the inherent antibacterial activity of colostrum, highlighting that colostrummanagement practices have important implications for bacterial growth.

Summary: This project evaluated the effect of postharvest thermal processing on the growth-promoting or -inhibiting activity of bovine colostrum. Colostrum was collected aseptically from Holstein cattle (n = 7), and aliquots were submitted to 3 treatments: cooling at 4°C only (RW), freezing at -20°C (FR), or heat treatment at 60°C for 60 minutes (HT). Colostrum replacer (CR) was included as a control. The day after treatments were applied, each sample was inoculated with a bovine gastrointestinal strain of Escherichia coli and incubated together at 37°C for 24 hours. Growth of E. coli was quantified on MacConkey agar plates immediately after inoculation (0 hours) as well as at 2, 4, 6, 8, and 24 hours. The resulting growth curves showed that cooled and freeze-thawed colostrum inhibited growth of E. coli, whereas heat-treated colostrum and colostrum replacer did not. These findings show that HT reduced the inherent antibacterial activity of colostrum, highlighting that colostrummanagement practices have important implications for bacterial growth.

•Raw colostrum had growth-inhibiting activity against a fecal strain of E. coli.•Freezing colostrum preserved its growth-inhibiting activity.•Heat treatment (60°C for 60 min) reduced colostrum's growth-inhibiting activity.

Raw colostrum had growth-inhibiting activity against a fecal strain of E. coli.

Freezing colostrum preserved its growth-inhibiting activity.

Heat treatment (60°C for 60 min) reduced colostrum's growth-inhibiting activity.

Providing newborn calves with sufficient high-quality colostrum is a critical calf-management strategy to support health, survival, and future productivity. Unfortunately, colostrum may also serve as a fomite for disease when contaminated with bacteria at harvest, during storage, or during reheating before feeding. Thermal processing, including heat treatment (HT; 60°C for 60 min) and freezing (FR; −20°C), are common strategies to manage bacterial contamination. Although both strategies maintain IgG concentrations, they destroy colostral immune cells, and HT is known to decrease the concentration and activity of certain bioactive factors such as complement and IgA. We hypothesized that HT and FR would influence the inherent antibacterial properties of bovine colostrum compared with unprocessed, refrigerated colostrum (RW). Our objective was thus to compare growth of Escherichia coli in RW, HT, and FR bovine colostrum. Sterile colostrum samples were collected from Holstein cows (n = 11) on a commercial dairy in New York State and divided into 4 aliquots. One aliquot was submitted for bacterial culture to exclude samples with any bacterial growth (n = 4). The remaining 7 samples were used for the experiment. Aliquots were either RW (4°C for 20 h; n = 7), HT (60°C for 60 min, then 4°C for 19 h; n = 7), or FR (−20°C for 20 h; n = 7). Immediately before inoculation of samples for a bacterial kinetics assay, a dried bovine colostrum-based replacer (CR; Ultra Start 150, Sav-A-Calf, Chilton, WI) was prepared from a single bag according to package instructions as a nutrient-rich control (n = 7). The prepared CR aliquots underwent bacterial culture to ascertain the absence of bacterial growth before inoculation. To simulate contamination that may occur on-farm, all samples were inoculated with 104 cfu/mL E. coli (WM1; 060913 P0lA, isolated from bovine feces) and tested in a bacterial kinetics assay at 37°C. At 0, 2, 4, 6, 8, and 24 h, growth of E. coli was quantified on MacConkey agar plates inoculated at 37°C. Relative to colostrum replacer, growth of E. coli was lower in RW and FR colostrum from 4 to 24 h and lower in HT colostrum at 6 and 8 h. These results demonstrate inherent microbial growth-inhibiting activity in RW colostrum and suggest that FR better preserved this activity than HT. Our study contributes to understanding the effect of thermal processing on antimicrobial properties of colostrum with the goal of helping to inform colostrum-management strategies for dairy producers. Specifically, our data emphasizes the need to practice hygiene during all steps of the colostrum harvest and storage process, including after completion of HT, because this renders colostrum more susceptible to bacterial growth. Similarly, the cooling and heating steps of stored colostrum should be rapid to minimize growth of E. coli.