Food Analysis – Pathogenic Bacteria in Food

Which are the relevant germs found in food This is so critical for our health because certain microorganisms in our food can cause foodborne diseases (colloquial: food poisoning). There are four types of microorganisms: bacteria, viruses, fungi, and protozoa. Amongst these are pathogenic (causing diseases) and non-pathogenic (harmless) organisms.

This article will focus on the most frequently occurring pathogenic bacteria in food:

Salmonella: egg products, meat products, chocolate, spices Staphylococcus: meat products, egg- and milk products, pasta products Listeria: meat products, milk products, fish, vegetable Escherichia: meat, raw milk, faecal contamination (of water) Which characteristics do the principle foodpathogenic bacteria have? Salmonella

Salmonella (two species: S. enteritidis and S. bongori [and further subspecies]) are chemotrophs (i.e. obtaining their energy from oxidation and reduction reactions) that live on organic sources. They are able to generate ATP with and without oxygen (aerobically and anaerobically) which is an enormous advantage. Salmonella enteritidis is found in homeothermic (“warm-blooded”) animals including humans while S. bongori specify on poikilothermic (“coldblooded”) animals. They usually invade the intestine, often causing gastroenteritis (symptoms: diarrhea, abdominal cramps, vomiting). The diseases caused by Salmonella infections are called salmonelloses. The main source of Salmonella are animals. The animals do not get diseased themselves, however, the bacteria is passed on to humans when eating raw meat or eggs.

One promising new way to reduce Salmonella in food production is to spray the food with a bacteriophage cocktail (bacteriophage = virus that specializes on bacteria). This method (according to a study conducted in 2013) leads to a significant decrease in the concentration of Salmonella . In the kitchen, please practice good hygiene and cook meat thoroughly (especially chicken), do not store food prepared from uncooked meat or eggs.

Staphylococcus

Staphylococcus aureus is also found in the upper respiratory tract and on the skin. It does not require oxygen to grow. Even though some S. aureus subspecies are part of the human microbiota, some can cause panaritium and foodborne diseases. They are commonly known to cause nosocomial infections. They are especially dangerous because most strains have developed a penicillin-resistance, and some even have a methicillin-resistance (MRSA). In food, S. aureus can cause gastroenteritis. This is due to staphylococcal enterotoxins produced by S. aureus.

Listeria

The notoriety of Listeria is due to just one of the twenty known species: Listeria monocytogenes. It causes listeriosis, which has a fatality-rate of around 20 % and that may cause a gastroenteritis. Listeria populations can grow at low temperatures, with or without oxygen and over a wide range of pH and salt concentrations which makes it highly concerning for the food industry.

Escherichia

Escherischia coli bacteria occur naturally in the intestines of warm-blooded animals (including humans) and are responsible for helping blood to congeal as well as protecting the intestine from pathogenic bacteria. However, they can also get into the environment after being excreted and can then cause damage when transferred faecal-orally (for example through unclean food). E. coli is one of the main causes of urinary tract infections. The strains EHEC (enterohemorrhagic E. coli ) and STEC (Shiga toxinproducing E. coli ) are especially serious and may cause diarrhoea.

Which methods are used in the lab to detect bacteria in food?

The traditional method of germ detection and identification in food is the triple sugar iron test (TSI) . The germs are grown on a media containing lactose, sucrose, glucose, sodium thiosulfate, ferrous sulphate, agar and phenol red (a pH-sensitive colour). They are then identified due to colour-change and biochemical characteristics. In the case of Salmonella, the ferrous sulphate indicates the hydrogen sulphide which is produced by most salmonella types.

A genotyping method to detect germs in food is real-time PCR using extracted germ DNA. The DNA is then multiplied and amplified so that it can be studied in detail. This is faster than the traditional method (24 hours instead of 4 to 5 days) and operates at approximately 100 % diagnostic accuracy. However, it remains expensive and requires a high level of expertise.

A chemotaxonomic method is MALDI-TOF MS (Matrix-assisted laser desorption ionization time-of-flight mass spectrometry). It is based on protein analysis and provides a more specific identification of the germ species than the traditional method. Similarly to genotyping methods, MALDI-TOF MS can sometimes be integrated into routine analyses (moderate costs). Additionally, it only takes 16 hours to analyse any food-associated types of microorganisms.

An enzyme-linked immunosorbent assay (ELISA) measures the concentration of protein antigens and antibodies. An advantage of ELISA is the quantification of the signal. With ELISA the antigen can be detected (direct method) or bind to an antibody which then detects an antigen (indirect method). The indirect method is often preferred since the signal strength is enhanced and, contrary to the direct method, not every antibody has to be marked for the specified antigen.

Internal transcribed spacer (ITS) – Sequencing focuses on the internal transcribed spacer which is a nucleotide sequence between different subunits in ribosomes. ITS-sequencing is often used because the ITS often mutates, thus indicating the species of germ. Fungi species are particularly well identified using ITS. The extracted ITS-sequence is then amplified with the PCR method.

EXERCISE: 1. Which one is the most common germ within food analysis?

a. Salmonella b. Escherichia c. Staphylococcus d. Listeria

2. Food analysis for bacteria is most often made with an ELISA

a. True b. False

You’ll find the answer on the next page.

BIBLIOGRAPHY 1. http://dict.leo.org. 2. Nielsen Suzanne S.: Food Analysis, 5th edition, Springer Verlag 2017. 3. Spricigo et al.: Int J Food Microbiol 165(2):169, 2013. 4. www.dict.cc. 5. www.webster-dictionary.org.

CARMEN LOBITZ Carmen Lobitz ist freie Trainerin, Lehrerin und MTA. Sie gründete 1998 in Berlin lobitz seminare, die unter anderem Englischseminare für Health Professionals anbieten. Kontakt: mail@lobitz-seminare.de www.lobitz-seminare.de

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Lösung für Exercise: Answer: 1. a–d Answer: 2. b

Food Analysis – Pathogenic Bacteria in Food

Which are the relevant germs found in food

Which characteristics do the principle foodpathogenic bacteria have?

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Food Analysis – Pathogenic Bacteria in Food

Which are the relevant germs found in food

Which characteristics do the principle foodpathogenic bacteria have?

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