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Foodborne Illness: What Causes Microbiological Contamination?

microbiological contamination of food

The increasing number of illnesses and deaths from foodborne pathogenic bacteria in the last few years has caused federal and state public health and regulatory agencies to give microbiological contamination a higher priority in their educational and regulatory efforts.

Dr. Sanford Miller, former director of the Food and Drug Administration’s Center for Food Safety and Applied Nutrition (CFSAN), has said that microbiological contamination has assumed a higher priority than many food chemical issues. He felt that 99% of the 1,600 food chemicals listed in the FDA’s “Red Book” have a safety margin of 1,000 or more above their no known effect level and that more emphasis should be placed on the microbiological hazards which develop when sanitation efforts are relaxed in times of economic constraints or when too much reliance is placed on safety measures such as pasteurization.

Recent outbreaks associated with both E-coli (O157:H7) and with Listeria monocytogenes in the meat industry have resulted in the recall and destruction of millions of pounds of meat and a regulatory directive issued specifically to address the control of Listeria monocytogenes in deli meats and hot dogs.

In addition to the serious health impacts to consumers, the economic impact of microbial incidents is substantial. Economic losses to food processors can be not only immediate, but losses to food processors can also be chronic:

  • Food spoilage losses
  • Food handler illness
  • Sampling and control costs
  • More costly processing techniques

As can losses resulting from an outbreak:

  • Product recall
  • Reduced consumer demand
  • Investigation costs
  • Clean up and plant closing costs
  • Liability suits
  • Food handler illness

Lawsuits associated with a 2017 listeriosis cheese case in the state of California totaled more than $800,000,000 and the manufacturer has gone bankrupt. The cost incurred from pathogenic contamination on the farm and in food service establishments is also of great concern. With all of these sectors of the food industry involved, it is easy to see how several billions of dollars are lost each year to the problems connected with these microorganisms, not to mention the physical suffering and deaths.

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What Are Serious Food Pathogens?

Pathogens are all bacteria that are unicellular, microscopic organisms present in nearly all environments. Various bacteria can tolerate temperatures ranging from around freezing up to 100°C—maximum temperature above which growth does not occur is 190°F. Some can survive outside of these temperatures by forming spores and becoming active again when conditions are more favorable. Many terms are used to describe and classify these organisms.

Physical Description

In appearance, the pathogens fall into three cell types:

  • Spherical (use the suffix “coccus” and can occur as single spheres, in chains, or clusters)
  • Rod-like (called bacillus and appear cylindrical or resembling a long ellipsoid)
  • Spiral (called spirillum and appear as long spiral or curved shapes)

The cells may also be called motile if they are capable of movement using tails (flagella).


Food pathogens live in a variety of locations and environments:

Table 1: Food Pathogens and Corresponding Environments
Aerobicbacteria grow on surfaces exposed to oxygen
Anaerobicbacteria grow below the surface or in oxygen-free environments
Enteroa prefix referring to the intestine
Facultative Anaerobecan grow in the presence of oxygen
Mesophileshave optimum growth temperatures around 25-40°C
Psychrophileshave optimum growth temperatures below 15°C with a range of 0-30°C. They can live in foods stored at refrigerator temperatures.
Psychrotrophspsychrophiles that grow quite rapidly at refrigerator temperatures
Thermophiles grow best at 45-60°C with a range up to 85°C


Pathogens react with a number of reagents that are useful in characterizing their species.

Gram Negative/Positive

A term that refers to the bacteria’s reaction to a staining technique developed by Christian Gram, a Danish scholar, in 1884. The retention of the crystal violet color is positive while a loss of this color and retention of the red safranin color is negative. This reaction can vary during the life cycle of some bacteria and these are termed gram variable.


Reactions are sensitive and specific for each type of organism. Each cell has cellular, capsular (flagellar H) and cell wall (somatic O) antigens which make them unique even in the same species. The various strains of the genus Salmonella are typed in this manner.

Other Means of Differentiation

Other methods of differentiation amongst pathogens include:

  • Nutritional requirements, i.e., organic carbon (heterotrophs), carbon dioxide (autotrophs), vitamins, amino acids, certain chemicals or elements
  • Products of metabolism, i.e., gases, alcohols, acids, indole or toxins
  • Spore formers are bacteria that form a small, thick-walled resistant body within the cell, enabling them to survive in a dormant stage until conditions are favorable for growth. Bacillus anthrasis (causing anthrax) and Clostridium (causing tetanus and botulism) are examples of spore formers.

Bacteria can be quite beneficial when regarded in the context of food fermentations, antibiotic production, genetic engineering, etc. However, bacterial pathogens can produce harmful, even grave consequences when proper control measures are ignored. Food poisoning caused by bacterial contamination occurs through two general modes.

One is an infectious or invasive mode whereby the pathogen causes infections in the intestinal tract of the host. Salmonella and Shigella are examples of this type of bacteria poisoning. The other mode is through intoxication caused by toxins that are produced by certain bacteria and then ingested by the victim. Examples of this type of pathogen are Staphylococcus aureus, Clostridium perfringens, and Clostridium botulinum.

Moving Toward Safer Foods

In 1983, the FDA began a pathogen surveillance program aimed at determining the prevalence in the food supply of the twelve microorganisms most likely to produce foodborne illness.

Table 2: The 12 Microorganisms Most Likely to Cause Illness
SalmonellaClostridium perfringensVibrio cholera
StaphylococcusAeromonas hydrophilaVibrio parahaemolyticus
Bacillus cereusEscherichia coliVibrio Vulnificus
Campylobacter jejuniListeria monocytogenes Yersinia enterocolitica

Since that time, several of these microorganisms have been responsible for many illnesses and deaths. Food manufacturers looking to keep the public safe, and protect their own pocketbooks, would do well to heed the stringent warnings of the regulatory community. As we've seen before, the fallout—whether it's finance-, public relations-, or public safety-related—from a single microbial incident can be devastating.

For more specific information, check out our second blog post in the series, Microbiological Contamination (Part 2): Which Pathogens Do the Most Damage?

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