HSBTE DMLT IInd Semester Bacterial Pathogenicity. Introduction, pathogenicity & infection. Sources of infection. Mode of spread of infection. Types of infection .

UNIT III: Bacterial pathogenicity

Introduction, pathogenicity & infection. Sources of infection, Mode of spread of infection, Types of infection

1. Introduction: The scientific study of bacteria is called bacteriology. Bacteria present on or inside the human body and rarely cause disease are called bacterial flora. These bacteria may be divided into three groups. These are symbionts, commensals, and opportunists. Bacteria present inside the living body and are beneficial for the living body are called Symbionts bacteria. eg. enteric bacteria that are found in the intestine and synthesize vitamin K and some members of the vitamin B complex. Bacteria present on the skin or on mucus membranes and neither harmful nor useful for the living body are called Commensals bacteria. Bacteria present on or inside the living body and produce disease during immunity weakness are called opportunist bacteria. eg. E.coli. , M. tuberculosis, Staphylococcus in the nose, etc.

2. Pathogenicity: A microorganism that develops disease inside a living body is called pathogen. The ability of a pathogen to develop disease in the living body is called bacterial pathogenicity. Disease disease-producing power of pathogens is called virulence. Virulence is defined as the degree of pathogenicity of the pathogen. Or. The extent of damage by microorganisms in the patient is called virulence. It is a measurement of pathogenicity. eg. opportunist bacteria have low virulence.

Factors affecting pathogenicity/virulence of pathogens

• i. Capsule: Host cells cannot engulf capsulated bacteria by phagocytosis.

• ii. Pilli: Pilli helps pathogen to adhere to the host cells.

• iii. Toxin: Toxins are metabolic wastes of pathogens that cause disease in the host cells. Toxins may be endotoxin or exotoxin. Gram-negative bacteria have endotoxin. The toxin released after the death of a pathogen is called endotoxin. Mainly gram-positive bacteria release exotoxin. Toxin secreted continuously from the pathogen is called exotoxin. It is mainly associated with gram-negative bacteria.

• iv. Extracellular enzymes: Enzymes secreted by cells into the external environment are called extracellular enzymes. Some pathogens secrete extracellular enzymes that cause disease. eg. streptokinase secreted by streptococcus protects streptococcus from penicillin antibiotics. Coagulase released by staphylococcus protects staphylococcus from phagocytosis.

• v. Transmission route: Pathogenicity and virulence of pathogen also depend upon the route of infection. eg. Vibrio cholarae causes cholera if enters the body through the oral route. It cannot cause cholera if enters the body through any other route. Staphylococcus aureus causes pneumonia if enters the lungs. It causes food poisoning if enters into GIT.

• vi. Pathogens number: Entry of a large number of one type of pathogens develops the disease. A small number of pathogen is killed by the immunity of the host.

• vii. Host susceptibility: Poor health, and low immunity of the host make the host susceptible to disease.

3. Infection: The invasion and multiplication of pathogenic microorganisms inside a living body is called infection. Following are the various terms related to infection:

• i. Immunity: The ability of a living body to prevent the multiplication of microorganisms inside the body and to resist the effect of toxins on the body is called immunity. Thus all infection does not lead to the development of disease.

• ii. Invasiveness: Pathogen's ability to enter into tissue is called invasiveness.

4. Sources of infection: Places or objects where pathogens survive are called sources of infection. Organisms that carry pathogens that infect human is called vectors. Vectors that allow pathogens to multiply inside their body are called biological vectors. eg. Malarial parasite(Plasmodium) in mosquito. Vectors that carry pathogens on their body are called mechanical vectors. eg. bacilli on flies' bodies to cause bacillary dysentery.

Route of infections: Pathogens enter into the body through the respiratory tract eg. tuberculosis. Gastrointestinal tract (GIT) eg. Typhoid, Cholera, Dysentery. Skin eg. malaria, filarial. and genitourinary tract eg. Gonorrhoea, syphilis, AIDs, UTI.

Following are the sources of infection: Humans, Higher and lower animals, birds, insects, food, soil., contaminated non-living surfaces, and contaminated air.

• i. Human: Humans are carriers of various pathogens. Both diseased and healthy humans act as carriers for pathogens. Humans may be temporary carriers, chronic carriers, contact carriers, or paradoxical carriers. Humans carrying pathogens from carriers other than human is called paradoxical carriers. A human who carries a pathogen from any other infected person is called a contact carrier.

• ii. Higher and lower animals: Animals also carry pathogens. Disease due to pathogens from animal sources is called zoonotic disease. eg. Rabies from dogs, plague from rats, anthrax from cattle, tetanus from horse

• iii. Birds: Birds are a source of several bacterial diseases, fungus diseases, and viral diseases. Infection in humans from birds is called bird disease or aviation disease. eg. viral diseases like bird flu (avian influenza), and fungal diseases like cryptococcal disease. Bacterial disease like Salmonellosis.

• iv. Insects: Malaria from Mosquito, Bacillary dysentery from flies.

• v. Food: Food carries a pathogen called contaminated food that spreads infection. eg. Food poisoning.

• vi. Soil and water: Pathogens in spore form are present in soil. eg. Tetanus spores.

• vii. Non-living surfaces: Non-living sources especially in hospitals carry pathogens. eg.  enterococcus, streptococcus, etc.

• viii. Air: Air carries pathogens that develop airborne diseases.

5. Mode of the spread of infection: Pathogens are transferred from the source of infection to the host through several modes. These modes can be divided into two groups

A. Direct transmission: Transfer of pathogens from person to person is direct transmission. Infection from person to person is called direct infection.

• i. Direct contact transmission: eg. Gonorrhoea transmitted through sexual contact. The mother transmits the infection to the fetus through the placenta. Chickenpox, ringworm, headlice, and conjunctivitis are transmitted through direct skin contact.

• ii. Droplet transmission: Sneezing, speaking, and coughing spreads infection through droplets. eg.. viral infection.

• iii. Body fluids/ blood; Examples of body fluids are tears, saliva, urine, breast milk, vaginal fluid, semen, etc. Contact of infected body fluid or infected blood with mucous membrane or blood of an uninfected person transmits the infection. eg AIDs, UTI, hepatitis, etc.

B. Indirect transmission:

The indirect spread of infection includes the following:

• i. Airborne transmission: Some pathogens remain suspended in the air for a longer duration. They infect a healthy person. eg. measles. influenza, tuberculosis,

• ii. Contaminated non-living objects: Touching contaminated surfaces infects a healthy person.

• iii. Vector-borne diseases: Vectors such as birds, animals, and humans carry pathogens. They spread disease. Such as Malaria through infected mosquito bites, rabies from infected dog bites, plague from infected rodents, and Anthrax from infected sheep. The transfer of infection from infected animals to healthy humans is called zoonosis.

• iv. Contaminated food and water: Food and water carry pathogens. Undercooked food and improperly caned fast food spread infection. Drinking contaminated water spreads infection. Such as bacillary dysentery, cholera, etc.

• v. Environments: Soil, water, and plants spread infectious diseases. Such as hookworm through contaminated soil, jaundice through water,

1. Types of infection

• i. Chronic infection: Infection that continues for a longer duration is called chronic infection.

• ii. Acute infection: Infection that continues for a shorter duration is called acute infection.

• iii. Subclinical infection: Infection without any symptoms in the patient is called subclinical infection.

• iv. Nosocomial infection: An infection that develops in patients during the hospitalization period is called nosocomial infection. It is also known as hospital-acquired infection.

• v. Iatrogenic infection: Infection in the living body after medical treatment or medical investigation is called iatrogenic infection.

• vi. Cross infection: Physical movement or transfer of harmful pathogen from one person, or object to another person or from part of the body to another part of the body is called cross infection.

• vii. Latent infection: The presence of the pathogen in inactive form inside a living body without any symptom of the disease is called latent infection.

• viii. Atypical infection: Infection without any symptom of the disease is called atypical infection.

• ix. Endogenous infection: Infection due to microorganisms of the normal flora of the living body is called endogenous infection.

• x. Exogenous infection: Infection due to the entry of a pathogen on or into a living body from the external environment is called exogenous infection.

• xi. Primary infection: Infection due to the entry of a pathogen on or into a living body is called primary infection.

• xii. Secondary infection: Infection during treatment of any other infection or after completion of any other infection treatment is called secondary infection.

• xiii. Re-infection: Infection developed by a pathogen after completion of treatment of that pathogen infection is called re-infection.

• xiv. Local infection: Infection in a specific part of a living body is called local infection.

• xv. Systemic infection: Infection in the physiological system of a living body is called systemic infection.