MICROBIOLOGY Study Guide - 2012

CHAPTER 13. PATHOGENIC MICROORGANISMS

13.1. PROPERTIES OF PATHOGENIC MICROORGANISMS

Food products serve as an excellent growth medium for numerous microorganisms. These include both Saprophytes—harmless environmental microorganisms inhabiting the external environment—and parasites, which are causative agents of various diseases. They are also referred to as pathogens (from the Greek patos meaning disease or suffering). Foods contaminated with pathogenic microorganisms can cause foodborne infections or food poisoning. Food production technologists, public health caterers, and food commodity experts must understand the Fundamental properties of pathogenic MICROORGANISMS AND THE sources through which they contaminate food.

Pathogenicity is a species-specific, heritable (i.e., genotypic) trait that reflects a microbe's potential ability to invade a host Organism and cause an infectious disease under appropriate conditions. A characteristic feature of pathogenic microbes is their Specificity of action—they induce strictly defined diseases with characteristic clinical signs.

Virulence (from the Latin virulentus meaning poisonous) is the degree or measure of pathogenicity. The unit of virulence is conventionally defined as the minimal lethal dose (DLM — dosis letalis minima), which represents the smallest number of microorganisms that causes 95–100% mortality in susceptible test animals. Typically, the median lethal dose (LD₅₀) is determined, which results in the death of 50% of the experimental animals.

Virulence can vary among microbes of the same species. Some strains are more pathogenic, while others exhibit weaker pathogenicity. Virulence is a phenotypic trait that can change in response to environmental fluctuations; it can be increased, decreased, and measured. For instance, cultivating pathogens on artificial growth media, passaging them through the bodies of non-susceptible animals, or exposing them to chemical, physical, or biological factors can reduce their virulence. Notably, A. Calmette and C. Guérin achieved a strain of the bovine tubercle bacillus with attenuated virulence through 230 consecutive passages on potato-Bile medium, which was subsequently used to produce the BCG vaccine. The artificial attenuation of pathogen virulence is termed attenuation, whereas strains of pathogens with artificially reduced virulence that retain immunogenic properties are called Vaccines. THE PRINCIPLE OF producing live vaccines was developed in 1880 by L. Pasteur.

Invasiveness is the ability of pathogenic microorganisms to penetrate mucous and Connective Tissues, enter the host's internal environment, and spread through its tissues. To overcome these barriers, pathogenic microbes secrete aggressins—substances that include Enzymes such as neuraminidase, hyaluronidase, collagenase, fibrinolysin, lecithinase, and others, collectively known as factors of invasion or spreading factors.

Neuraminidase cleaves sialic acids present in the surface protective mucus layer, altering membrane surface charge and cation transport, which ultimately disrupts the functional activity of Organs and tissues.

Hyaluronidase breaks down hyaluronic acid, a key component of Connective Tissue that provides structural strength and impermeability to microbes. The breakdown of hyaluronic acid reduces connective tissue resistance, thereby facilitating the spread of microbes throughout the body. Hyaluronidase is produced by gas gangrene bacilli, staphylococci, streptococci, and diphtheria agents.

Collagenase causes the breakdown of Muscle tissue and is found in the gas gangrene bacterium.

Fibrinolysin lyses fibrin clots that form in the human bloodstream during inflammatory responses, which would otherwise hinder the spread of microbes in the body. This enzyme is secreted by streptococci, staphylococci, and the plague bacillus.

Lecithinase—an enzyme belonging to the lipase group—hydrolyzes lecithins that constitute plasma and mitochondrial membranes. Lecithinase is produced by staphylococci and Bacteria of the genus Clostridium.

Toxin production. The most critical property of pathogenic microorganisms is their ability to synthesize toxic metabolic products known as toxins. Upon entering the bloodstream, these toxins are distributed throughout the body, causing systemic poisoning. Microbial toxins are broadly categorized into exotoxins and endotoxins. The ability to produce protein-based exotoxins is characteristic of both Gram-positive and Gram-negative aerobic and anaerobic bacteria. Endotoxins are primarily produced by Gram-negative bacteria.

Exotoxins are secreted by The Cell into the external environment. Being Proteins, exotoxins are generally thermolabile. Specifically, diphtheria toxin is inactivated at 60 °C within one hour, and tetanus toxin within 20 minutes. However, certain thermostable exotoxins exist that can withstand brief boiling; these include the toxins produced by the botulinum bacterium, Staphylococcus aureus, and perfringens bacillus (Clostridium perfringens). Exotoxins exhibit high specificity of action, targeting specific organs and tissues. For instance, botulinum toxin (Clostridium botulinus) affects cranial motor nerves, causing visual disturbances, swallowing difficulties, and respiratory paralysis. Diphtheria toxin targets The Heart muscle and Adrenal Glands. Many bacteria produce multiple toxins with distinct activities, such as hemolytic, neurotoxic, cytotoxic, or dermonecrotic effects. The synthesis of microbial toxins is governed by genes localized in Plasmids.

Endotoxins are tightly bound to the cell and are released only upon its lysis. They are located in the outer membrane of The Cell wall of Gram-negative bacteria and consist of lipopolysaccharides, proteins, and lipid A. Lipid A is composed of glucosamine, phosphate, and high-molecular-weight Fatty acids.

Unlike exotoxins, endotoxins are more resistant to elevated temperatures, less toxic, and exhibit lower specificity. Typically, when a host organism is affected, they elicit a uniform clinical picture: fatigue, shortness of breath, cardiovascular depression, and intestinal disorders. Small doses of endotoxin cause fever, whereas large doses induce hypothermia.

Certain microorganisms are capable of producing both exotoxins and endotoxins simultaneously, such as the cholera vibrio and hemolytic strains of Escherichia coli.