Scanning electron microscopy of Salmonella biofilms on various food-contact surfaces in catfish mucus

Authors: DHOWLAGHAR, N - Mississippi State University; BANSAL, M - Mississippi State University; SCHILLING, M - Mississippi State University; NANNAPANENI, R - Mississippi State University

Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2018
Publication Date: 3/28/2018
Citation: Dhowlaghar, N., Bansal, M., Schilling, M.W., Nannapaneni, R. 2018. Scanning electron microscopy of Salmonella biofilms on various food-contact surfaces in catfish mucus. Food Microbiology. 74(9):143-150.

Interpretive Summary: Salmonellosis outbreaks have been linked to the consumption of seafood in the United States such as molluscs, crustaceans and finfish products. A Salmonella enterica ssp. enterica serovar Hadar outbreak with 10 cases that was reported from a New Jersey restaurant was linked to catfish. Due to this outbreak, Centers for Disease Control and Prevention (CDC) has listed catfish as a vehicle for the spread of Salmonella. In the present study, we determined the survival of Salmonella isolates in the presence of catfish mucus extract for up to 63 d at two temperatures (10 °C and 22 °C). We have also observed the biofilm formation by Salmonella on four food-contact surfaces by scanning electron microscopy in the presence of catfish mucus. In addition, the surface properties, surface roughness and surface energies were also determined using contact angle measurement and atomic force microscopy. Our findings indicate that Salmonella can grow and survive for longer periods in the presence of catfish mucus concentration as low as 15 µg/ml. The surface wettability, surface free energy, surface roughness and SEM images substantiated the bacterial attachment and biofilm formation on different food-contact surfaces. These findings will aid in developing proper cleaning and sanitation procedures in the catfish processing environment to reduce the contamination and persistence of Salmonella biofilms.

Technical Abstract: The objective of this study was to determine the growth and survival of Salmonella enterica in the presence of high and low concentrations (375 µg/ml and 15 µg/ml) of catfish mucus extract at 10 °C and 22 °C for 63 days. The second objective of this study was to investigate the biofilm formation of Salmonella enterica serovar Blockley (7175) in catfish mucus extract for 48 h at 22 °C on four food-contact surfaces and to observe the biofilm populations using Scanning Electron Microscopy (SEM). The surface properties, surface roughness and surface energies were determined using contact angle measurement and atomic force microscopy. In 375 µg/ml of catfish mucus extract that was inoculated with 3 log CFU/ml, the growth of Salmonella counts were increased to a maximum of 6-7 log CFU/ml at 10 °C and 7-8 log CFU/ml at 22 °C in 7-14 d and decreased by 1-2 log CFU/ml from these peak levels at both 10 °C and 22 °C from 21-63 d. In 15 µg/ml of catfish mucus extract, Salmonella counts were in the range of 4-5 log CFU/ml at 10 °C and 5-6 log CFU/ml at 22 °C over 7-63 d of storage. By contrast, Salmonella counts were non-detectable in the absence of catfish mucus by 21-28 d of storage at 10 °C or 22 °C. The biofilm counts of S. Blockley (7175) on a stainless steel surface were 4 log CFU/cm2 and 5.5 log CFU/cm2 in 15 µg/ml and 375 µg/ml of catfish mucus extract respectively after 48 h incubation at 22 °C. SEM revelead that biofilm formation by S. Blockley (7175) was less in 15 µg/ml than 375 µg/ml of catfish mucus extract on stainless steel. In addition, SEM indicated that the visible biofilms were least on buna-N rubber as compared to stainless steel, polyethylene and polyurethane surfaces. Contact angle and atomic force microscopy confirmed that buna-N rubber was highly hydrophobic with low surface energy and low roughness when compared to other three surfaces. These findings indicate that Salmonella can utilize catfish mucus as a nutrient source to survive for longer periods and promote biofilm formation for its persistence on different food-contact surfaces.