Microbiology of Bacteria, Fungi, and Viruses in Food

Posted on Oct 31, 2024 in Biology

Bacteria

Bacteria are unicellular, prokaryotic organisms, ranging from 0.2 to 5 microns in size. Their genome consists of a single, large, circular, closed DNA molecule (nucleoid). The cytoplasm contains ribosomes and reserve products. Bacteria possess a plasma membrane and a bacterial wall made of peptidoglycan. The Gram stain differentiates bacteria into two groups: Gram-positive and Gram-negative, based on cell wall characteristics. Bacteria can harbor exogenous genetic material (plasmids and bacteriophages), are sometimes surrounded by a polysaccharide capsule, and certain Gram-positive bacteria (e.g., Bacillus and Clostridium) can form resistant endospores.

Identification of Bacteria

  • Morphological Characteristics: Cocci or bacilli shape, presence of spores, Gram stain result, motility, optimal growth temperature, respiratory type, and macroscopic appearance of colonies.
  • Metabolic Characteristics: Differential media containing specific substrates are used to study bacterial metabolism. Metabolic byproducts cause pH changes in the media, detected by color-changing reagents. Microorganisms with similar metabolic traits are called biotypes.
  • Serotyping: Immunological detection of bacterial antigens, such as the O antigen in the lipopolysaccharide membrane, the Vi capsular antigen, and the H flagellar antigen. Microorganisms sharing antigenic characteristics are called serotypes. Example: Salmonella enterica serovar Typhimurium (O: 4,5, H: i, H: 1,2).

Fungi

Fungi are heterotrophic eukaryotes that utilize glycolysis and aerobic respiration. They are widely distributed and can be:

  • Unicellular (yeasts): Reproduce asexually by budding or fission and sexually by cytoplasmic fusion (3 to 15 microns).
  • Multicellular (molds or mildew): Reproduce asexually by sporulation and sexually by cell fusion at hyphal tips.
  • Dimorphic (can switch between yeast and mold forms).

Fungi are part of the normal flora of many foods, thriving in environments less suitable for bacteria, such as low pH, high sugar or salt concentrations, and low water activity. Most are strict aerobes, some yeasts are facultative anaerobes (glycolysis and alcoholic fermentation), and very few are strict anaerobes. Fungi have typical eukaryotic organelles, a true nucleolus (RNA synthesis), a plasmalemma (glycoproteins, phospholipids, and ergosterol), a cell wall (chitin, glucans, polysaccharides, and polypeptides), and sometimes a polysaccharide capsule. The cell wall contributes to fungal virulence. Some molds produce secondary metabolites called mycotoxins during exponential growth (e.g., aflatoxin in Aspergillus flavus and Aspergillus parasiticus; ochratoxin and patulin in Penicillium species; ergotamine and ergotoxine in Claviceps purpurea). Aflatoxins are potent animal carcinogens.

Viruses

Viruses are obligate intracellular parasites that lack mechanisms for energy production and protein synthesis. They replicate within bacterial (bacteriophages) or eukaryotic cells, exhibiting high host specificity. Viral components are assembled rather than replicated by division. A virus consists of a DNA or RNA genome and polymerases, enclosed in a capsid made of glycoprotein capsomeres. The genome and capsid together form the nucleocapsid. Viruses with only a nucleocapsid are called “naked,” while those with an additional lipid envelope (derived from host cell membranes) are called “enveloped.” Nucleocapsids can be icosahedral or helical. Viruses are classified based on their nucleic acid type (RNA or DNA), nucleocapsid shape (icosahedral or helical), and presence or absence of an envelope (naked or enveloped). Viruses range from 30 to 300 nanometers in size and are visible only with an electron microscope. Viruses can contaminate food but cannot replicate in it, thus they do not cause food spoilage but pose health risks.

Viral Replication

Viral replication involves: target cell recognition (using viral adhesion proteins), attachment, penetration (naked viruses via receptor-mediated endocytosis, enveloped viruses via membrane fusion), uncoating, macromolecular synthesis (viral DNA or RNA and capsid proteins), virus assembly, and release (budding for enveloped viruses or cell lysis for naked viruses).

Viral Infection Steps

  1. Virus acquisition (through skin or mucous membranes)
  2. Primary infection
  3. Incubation period (prodromal phase: nonspecific symptoms like fatigue)
  4. Disease symptoms (due to tissue damage and systemic effects caused by the virus and the immune response)

Viruses of Interest in Food Microbiology

  • Hepatitis A: Transmitted through fecal-oral route, often via contaminated food or water. Causes systemic disease.
  • Norovirus: Causes gastroenteritis, commonly associated with traveler’s diarrhea and contaminated food or water. Affects both children and adults.