PHARMACEUTICAL MICROBIOLOGY - V. A. Galynkin - 2015

PART I. MICROBIAL BIOLOGY

CHAPTER 7. FUNDAMENTALS OF MICROBIAL PATHOGENICITY. INFECTIOUS DISEASES

7.1 PATHOGENICITY AND VIRULENCE

Pathogenic (from Greek pathos meaning suffering or disease) or disease-causing microorganisms are capable of causing diseases in humans, animals, and plants. Bacteria, Fungi, Protozoa, and Viruses can all be pathogenic.

Opportunistic microorganisms are those that cause disease only under conditions unfavorable to the host Organism. For humans, such conditions may include hypothermia, radiation, malnutrition, intoxication, another underlying disease, etc.

Pathogenicity is the potential ability of certain microbial species to cause infectious diseases; it is a species-specific, genetically determined trait that results from the evolutionary adaptation of a microorganism to a parasitic lifestyle.

Virulence is the degree of pathogenicity of a specific strain, representing its individual characteristic that can change under the Influence of Environmental conditions. Virulence can be increased through passages (successive infections) in susceptible animals, or attenuated by exposure to factors unfavorable to the microorganism (immune sera, biocides, etc.). The latter approach is used to obtain avirulent vaccine strains. Additionally, virulence can be modified via genetic recombination Methods.

The following indicators are used to characterize virulence:

— Dcl (Dosis certa letalis) — the dose (number of microbial Cells) that causes the death of all infected animals;

— Dlm (Dosis letalis minima) — the dose that causes the death of approximately 80% of infected animals;

— Dl₅₀ — the dose that causes the death of 50% of infected animals, determined statistically.

7.2 Defense and Aggression Factors

A microorganism's pathogenicity and virulence are linked to the genetically determined features of its Structure and METABOLISM. Virulence genes form clusters (pathogenicity islands) on Chromosomes and Plasmids that are capable of horizontal transfer. Similar virulence genes are found in taxonomically distant species. Pathogenicity and virulence are determined by the microorganism's ability to evade host defense mechanisms and produce substances that dictate its invasiveness (The ability to spread within the body) and aggressive properties. All these features are collectively known as virulence factors, or defense and aggression factors. There are various ways in which pathogens evade host defense mechanisms:

✵ Integration of the genome of certain viruses into the host chromosome, followed by vertical transmission (inheritance).

✵ Localization of parasites (viruses, agents of tuberculosis, leprosy, brucellosis, leishmaniasis, etc.) inside cells of The Immune System (macrophages, lymphocytes).

✵ Synthesis of immunosuppressants, i.e., substances that inhibit the synthesis and activity of Antibodies, Complement, Lysozyme, and immunocompetent cells.

✵ Alteration of the infectious agent's surface Antigens to closely resemble host antigens (molecular mimicry).

✵ Antigenic variation of the parasite throughout the infectious process, driven by genetic recombination involving phages, plasmids, Transposons, and IS-elements, which allows microbes to escape the host's immune system.

✵ Formation of resting stages (spores, cysts) that are resistant to environmental influences.

✵ Special Cell surface features that provide cellular defense: a capsule in pathogenic Klebsiella, Clostridium, Yersinia, Streptococcus, and Bacillus anthracis; an outer membrane in Gram-negative bacteria; cord factor in Mycobacterium tuberculosis; Fc-receptor Proteins in staphylococci and streptococci, etc. The capsule protects microbial cells from phagocytosis and facilitates their attachment to body Tissues. Outer membrane lipopolysaccharides block antibodies and exhibit endotoxin properties. Cord factor (a lipid, trehalose dimycolate) promotes cell clumping and acid-fastness. Fc-receptor proteins, which non-specifically bind IMMUNOGLOBULINS, protect The Cell from specific antibodies, suppress phagocytosis and the Immune Response, and inactivate The Complement System.

✵ Pili (fimbriae) ensure Cell Adhesion and microcolony formation. Adhesion occurs via specialized proteins or Glycoproteins called adhesins, which are located on the cell surface—often on pili—and interact with Eukaryotic cells in a lectin-like manner involving carbohydrate-protein recognition. This interaction is Ligand-receptor-based, where the adhesin acts as the ligand and a corresponding carbohydrate structure on the host cell acts as the receptor. This biological recognition mechanism underlies the Specificity of both tissue infection by microbes and the functioning of the body's defense mechanisms.

✵ Motility is a crucial factor in invasion, as it facilitates the penetration of microorganisms into Cells and Tissues.

Toxins. Based on their localization, toxins are classified into exotoxins and endotoxins. Exotoxins are synthesized by the causative agents of tetanus, botulism, anaerobic infections, diphtheria, pertussis, cholera, plague, as well as certain species of Shigella, staphylococci, streptococci, Pseudomonas, and others. These are proteins produced by The Cell as inactive precursors; their activation occurs via Limited proteolysis mediated by microbial or host Enzymes. As a result of activation, toxins acquire the enzymatic activity of ADP-ribosyltransferase, which triggers a cascade of reactions leading to disrupted synthesis of cyclic AMP and, consequently, impaired Regulation of Protein Synthesis in the host cell. Many exotoxins exhibit selective effects on specific Organs and tissues: diphtheria toxin damages the Adrenal Glands and Heart Muscle, while tetanus toxin affects motor Nerve Cells. Exotoxins act on a susceptible organism in very small doses; for example, 1 ml of diphtheria toxin contains 10000 Dlm for a guinea pig (where Dlm of a toxin is its minimum dose capable of killing the test animal). Some exotoxins are heat-stable and resist digestive enzymes (e.g., botulinum and staphylococcal toxins). Treatment with formalin, which blocks the amino groups of the active center, leads to a loss of toxicity, a principle utilized in the preparation of toxoid Vaccines.

Endotoxins are tightly bound to the cell and are typically released into the environment only after cell lysis. They are usually Glycoconjugates (lipopolysaccharides, glycoproteins, Glycolipids) of The Cell wall, most commonly found in the outer membrane of Gram-negative bacteria. Lipid A plays a critical role in the toxicity of these substances. Endotoxins manifest their toxicity at significantly higher concentrations than exotoxins. They possess pyrogenic activity, which forms The basis of methods used to detect them, for instance, in injectable solutions. Endotoxins can activate the complement system and the Blood clotting system, and they affect the body's enzyme systems, disrupting Carbohydrate Metabolism, Liver function, etc. Endotoxin receptors are present on the membranes of platelets, macrophages, lymphocytes, and capillary endothelium. The effects of endotoxins depend on their concentration: at low doses, they can stimulate phagocytosis and other host defense reactions, which is why some endotoxins are used as immunomodulators (e.g., pyrogenal).

Pathogenic enzymes catalyze reactions leading to The formation of toxic products or the destruction of organism cells and tissues.

Clostridium perfringens lecithinase C (phospholipase) hydrolyzes cell membrane lecithin (phospholipid), damaging erythrocytes and other cells, and causing tissue necrosis.

Neuraminidase from the cholera vibrio, anaerobic infection agents, streptococci, Influenza virus, etc., hydrolyzes compounds containing sialic acids. These substances determine the viscosity of biological fluids, participate in cell aggregation, biological recognition processes, Intracellular Transport, and more; therefore, the action of neuraminidase can lead to the disruption of various body functions.

Fibrinolysin and hyaluronidase from streptococci and other microorganisms act as spreading factors, facilitating the penetration of microbial cells into body tissues. Hyaluronidase hydrolyzes hyaluronic acid, a complex mucopolysaccharide that provides viscosity to the intercellular substance. Consequently, this enzyme can be used in combination with medications to accelerate their penetration into tissues, eliminate scars, etc.

Staphylococcal and streptococcal hemolysins and leukocidins destroy erythrocytes and leukocytes.

Plasmacoagulase of staphylococci and other microorganisms is a peptidase that activates the Blood Coagulation SYSTEM through the catalytic conversion of prothrombin into Thrombin, ensuring the creation of a protective fibrin capsule around microbial cells.

Urease of pneumococci, klebsiellae, and yersiniae hydrolyzes amides with the Formation of the toxic ammonium ion.

Decarboxylases of anaerobic infection agents and other microorganisms catalyze reactions resulting in the formation of toxic amines.

7.3 Infectious Diseases

Infectious diseases are a group of disorders caused by pathogenic microorganisms, such as viruses, bacteria, and protozoa. A common feature of most infectious diseases is the possibility of transmitting the causative agent from a sick person to a healthy one and their potential for mass (epidemic) spread. As a result of interaction with the pathogen, a complex of physiological (adaptive) and pathological processes develops in the body, accompanied by a disruption of Homeostasis. Symbiotic relationships that are harmful to the host are referred to as antagonistic Symbiosis, the extreme manifestation of which is parasitism. Obligate Intracellular Parasites include viruses, rickettsiae, and chlamydiae.

The source of infection is the environment in which the disease-causing agent can live and reproduce under natural conditions. Diseases whose primary source is human are termed anthroponoses; those transmitted from animals are zoonoses; and diseases caused by microorganisms residing in Water, soil, and other environmental objects are sapronoses. Infections can originate from various sources (from a human or animal, as well as from contaminated environmental objects favorable for pathogen reproduction): soil can serve as a reservoir for pathogens of salmonellosis, dysentery, anthrax, tetanus, and anaerobic infections; water for intestinal infections, tularemia, and hepatitis A; and food products for intestinal infections, tuberculosis, brucellosis, scarlet fever, diphtheria, and food toxicoinfections.

The routes of entry of an infectious agent into the body are determined by its nature. Intestinal infection pathogens enter via the Mouth with water and food; respiratory ones through the respiratory tract; malaria, rickettsioses, encephalitis, AIDS, hepatitis B, and others are transmitted through the blood (insect bites, contaminated instruments, injection solutions); and dermatomycoses and Sexually Transmitted Diseases enter through the Skin and mucous membranes. Pathogenic agents of plague, anthrax, tuberculosis, diphtheria, and scarlet fever are capable of entering the body through any of the aforementioned routes. Some diseases can be transmitted vertically from mother to foetus. Transmission of the agent (Syphilis, Gonorrhea, typhoid and relapsing fever, Toxoplasmosis, staphylococci, etc.) can occur across the Placenta or during passage through the birth canal.

Periods of development of an infectious disease. The period from the moment of infection to the appearance of the first signs of the disease is called the incubation period. During this time, the pathogen multiplies, microbial toxins are synthesized, and host defense reactions develop. When a balance is established between the pathogen's aggression factors and the body's defense forces, the disease may proceed without overt symptoms (latently) or manifest no signs at all. Under conditions that reduce resistance (stress, hypothermia, malnutrition, etc.), microorganisms acquire the ability to exert a pathogenic effect.

The premonitory period of the disease, known as the prodromal period, is characterized by the manifestation of the very initial, nonspecific signs of illness (malaise, mild fever).

During the stationary period, the main symptoms characteristic of the given disease appear. Sometimes the disease may take an atypical course, for instance, under intensive antibiotic treatment.

In the period of convalescence (decline), a decrease in the intensity of pathological processes and the disappearance of clinical signs are observed. The decline of the disease concludes either with recovery or a transition to a chronic state. Following recovery, an individual may remain a carrier of the infectious agent.

Treatment tactics are structured According to the period of the infectious disease. Preventive measures are of great importance: adherence to Sanitary and hygienic rules, a healthy lifestyle, body hardening, as well as preventative vaccination.

Forms of Infectious Diseases

Depending on the source of the infectious agent, a distinction is made between exogenous infection (when the pathogen is introduced from the outside) and endogenous infection, which arises as a result of the activation of one's own microbiota due to a disruption in the relative constancy of its composition (dysbiosis) and The impact of external factors that reduce resistance. Endogenous infections include tonsillitis, appendicitis, cholecystitis, Osteomyelitis, pustular skin diseases, and others.

Based on their localization within the body, infectious diseases are subdivided into focal (local) and generalized (systemic). Under conditions unfavorable for the patient, a local infection can evolve into a generalized one, as seen in furunculosis, tuberculosis, syphilis, candidiasis, etc.

When microbes or their toxins spread throughout the entire body, this is referred to as bacteremia, viremia, septicemia, or toxinemia.

According to the type of infecting agents, a distinction is made between monoinfections and mixed infections. The latter feature a qualitatively different clinical course compared to monoinfections, and the interactions between the pathogens are quite variable. The most unfavorable for the patient are symbiotic relationships, such as those between trichomonads and gonococci, where bacteria inhabit protozoan cells, complicating the Treatment of the disease.

Repeated manifestations of a disease are characterized as a secondary infection when complicating a primary illness (e.g., bacterial Pneumonia following influenza or measles); reinfection—a repeated infection with the same species of microorganism after recovery, typical of sexually transmitted diseases; and relapse—the resurgence of a disease after Clinical Recovery, caused by persistent pathogens.

Depending on the duration of the infectious agent's stay in the body, acute, chronic, and persistent infections, as well as microbial carriage, are distinguished. Persistence is characterized by the long-term survival of microorganisms within the host organism. It is observed in hepatitis B, herpes, rubella, tuberculosis, malaria, toxoplasmosis, and others. Bacteria can persist in the form of L-forms and intact cells within body tissues and cells, including macrophages. The ability to persist in macrophages is inherent to all non-spore-forming microorganisms that cause anthroponotic infections, serving as one of their survival and species-preservation mechanisms. Persistence is facilitated by microbial defense and aggression mechanisms such as molecular mimicry, the ability to inactivate lysozyme, complement, immunoglobulins, and other vital protein molecules using extracellular proteases.

Microbial carriage is a form of interaction between the micro- and macro-organism in which no overt signs of the disease are observed. It is associated with the relative insusceptibility of the body or a low level of Immunity (tolerance), is characteristic of intestinal infections, scarlet fever, meningitis, malaria, poliomyelitis, etc., and arises in individuals who have recovered from the illness or in healthy contacts.

All carriers pose an epidemiological threat. The treatment of both persistent infections and microbial carriage aims to stimulate the body's immune system.

Depending on their outward manifestations, diseases are classified as overt or covert (latent, asymptomatic). Under conditions unfavorable to humans, a latent infection can progress into an overt one. Asymptomatic or atypical infectious diseases often emerge as a result of treatment with Antibiotics and other potent antimicrobial agents. The absence of characteristic symptoms complicates Diagnosis, thereby hindering the Selection of appropriate treatments and the Organization of anti-epidemic measures.

Based on their prevalence within a population, infectious diseases can be sporadic (occurring as isolated cases) or epidemic (surpassing the baseline level of sporadic incidence). The latter form of disease in animals is referred to as an epizootic. When an epidemic reaches exceptionally massive proportions, spanning countries and continents, it is termed a pandemic. Endemic infections are those whose causative agents persist in a specific geographic region over an extended period (e.g., tick-borne encephalitis, tularemia, malaria).

Epidemiology of infectious diseases.

Infectious diseases can be transmitted vertically (from one generation to the next) and horizontally (among unrelated members of a population). In the latter case, transmission may occur from a common source (such as water or food products) or directly from person to person, with each individual serving as a source of infection for others.

Infections originating from a common source are characterized by a sharp spike in morbidity, with similar incubation periods and clinical courses across all affected patients. Conversely, when an infectious agent is transmitted from a sick individual to a healthy one, the number of cases grows gradually, while the incubation period and disease progression depend on individual host susceptibility.

The Factors Determining the onset of an epidemic are:

a) the infectiousness of the pathogen (its ability to spread rapidly and overcome host defense mechanisms);

b) population density within a given region;

c) the number of susceptible individuals within the population.

A shift in even one of these factors can impact the likelihood of an epidemic outbreak. For instance, epidemic outbreaks of measles and chickenpox in early autumn among children returning to school after vacation are driven by the concentration of susceptible individuals in a single Setting. Preventive vaccinations reduce or prevent the potential for epidemics by decreasing the number of people susceptible to a given disease.