Bacterial Growth and Pathogenesis Lecture PDF

Microbial World and You; Bacterial Growth and Pathogenesis Natia Tamarashvili, PhD Bacterial Growth Physical Requirements – Temperature Psychrophiles Mesophiles Thermophiles pH Osmotic Pressure Acidophiles- Grow between pH 0 and 5.5. Examples: Ferroplasma, Thiobacillus Sulfolobus acidocaldarius, etc...

Microbial World and You; Bacterial Growth and Pathogenesis Natia Tamarashvili, PhD Bacterial Growth Physical Requirements – Temperature Psychrophiles Mesophiles Thermophiles pH Osmotic Pressure Acidophiles- Grow between pH 0 and 5.5. Examples: Ferroplasma, Thiobacillus Sulfolobus acidocaldarius, etc. thioxidans, Alkalophiles- Grow between pH range of 7.5 to 14. Examples: Thermococcus alcaliphilus, etc. Neutrophiles- Grow between pH 5.5 to 8.0 Examples: Lactobacillus acidophillus, Pseudomonas aerunginosa, etc. E. coli, Bacteria prefer media of pH near neutrality, and usually cannot tolerate pH values much below 4-5. The Requirements for Growth Temperature pH Oxygen Osmotic pressure Carbon Nitrogen, sulfur, and phosphorous Trace elements (iron, copper, zinc) Obligate (strict) vs. facultative “Obligate” (or “strict”) means that a given condition is required for growth. “Facultative” means that the organism can grow under the condition, but doesn’t require it. The term “facultative” is often applied to suboptimal conditions. For example, an obligate thermophile requires elevated temperatures for growth, while a facultative thermophile may grow in either elevated temperatures or lower temperatures. Binary Fission Binary Fission is a type of Asexual reproduction in single celled organisms Bacterial Doubling Time Escherichia coli Mycobacterium tuberculosis Mycobacterium leprae 20 minutes 18 hours 14 days The doubling time varies not only with the species, but also with the amount of nutrients, the temperature, the pH, and other environmental factors. Stages of Bacterial Growth Lag Log (logarithmic) Stationary Death Lag phase The first is the lag phase, during which vigorous metabolic activity occurs but cells do not divide. This can last for a few minutes up to many hours. An initial stationary phase represents the time from the moment of seeding the bacteria on the nutrient medium. bacteria absorb nutrients, synthesize enzymes and prepare for cell division. Logarithmic growth phase Phase of exponential (logarithmic) growth which is characterized by a maximal division rate and decrease in cell size. The log phase is always brief, unless the rapidly dividing culture is maintained by constant addition of nutrients and frequent removal of waste products β-Lactam drugs, such as penicillin, act during this phase because the drugs are effective when cells are making peptidoglycan (i.e., when they are dividing). Logarithmic or Exponential Growth Phase of exponential (logarithmic) growth which is characterized by a maximal division rate and decrease in cell size. The population of bacterial cells divide at a constant rate so that the total number of cells doubles with each division. Because one cell gives rise to two progeny cells, bacteria are said to undergo exponential growth (logarithmic growth). Bacteria Undergo Exponenti al Growth Stationary phase The stationary phase occurs when the nutrients in the liquid medium are used up and the concentration of toxic waste products from the metabolizing bacteria build up, the rate of division slows, such that the number of bacteria that are dividing equals the number that are dying. Death phase The final phase is the death phase, which is marked by a decline in the number of viable bacteria. The mas overcrowding occurs the concentration of toxic waste products continues to increase and the nutrient supply decreases.  Microorganisms die at a rapid rate. Types of Pathogens A microorganism is pathogenic if it is capable of producing a disease in an immunocompetent person. True Pathogens a) Highly Pathogens b) Less Pathogens Definitions Pathogenicity and Virulence Pathogenicity The ability of a microbe to cause disease This term is often used to describe or compare species. Virulence The degree of pathogenicity in a microorganism This term is often used to describe or compare strains within a species. A microorganism is pathogenic if it is capable of producing a disease in an immunocompetent person. True Pathogens a) b) Highly Pathogens Less Pathogens Virulence Virulence: defined as quantitative measure of pathogenicity, and is measured by minimum number of bacteria to cause the disease. LD 50 (The 50% lethal dose): Number of organisms required to kill 50% hosts. ID 50 (50% infectious dose): Number of organisms required to cause infection in 50% of host population. Virulence Organisms with lower LD 50 are more virulent than higher ID 50, as fewer bacteria are required to cause death or disease. Infectious dose of an organism required to cause disease varies greatly among the pathogenic bacteria. Shigella (ID 50 less than 100) and Salmonella (100,000) cause diarrhea by producing infection in GIT. Number of invading microbes LD 50 - Lethal Dose of a microbes or toxin that will kill 50% of experimentally inoculated test animal within a certain period of time ID 50 - Infectious Dose required to cause disease in 50% of inoculated test animals Example: ID 50 for Vibrio cholerea 10 8 cells (100,000,000 cells) ID 50 for Inhalation Anthrax - 5,000 to 10,000 spores ID for E.coli O157:H7 - < 10 cells 21 Infectious dose infective dose (ID) amount of a pathogenic agent that will cause infection in susceptible Subjects. The infectious dose of bacteria depends primarily on their virulence factors. Virulence factor: Infectious dose of bacteria depends upon virulence factor i.e. whether a bacteria has: Pilli Toxin Capsule TYPES OF BACTERIAL INFECTIONS The word infection does not have to be equated with disease. Bacteria cause disease by two major mechanisms: (1) toxin production and (2) invasion and inflammation. 24 Mechanisms of bacterial pathogenicity Invasiveness: the ability to invade tissues encompasses mechanisms for colonization (adherence and initial multiplication), production of extracellular substances which facilitate invasion (invasins) and ability to bypass or overcome host defense mechanism. 25 Types of Bacterial Infections Non communicable (e.g. botulism) Communicable, spreads from host to host via airborne, droplets and coughing ( e.g. Tuberculosis) Contagious, Highly communicable (e.g. Influenza) Asymptomatic / inapparent. Chronic carriers, the organisms continue to grow with or without producing the symptoms in the host. Epidemiological Classification of Bacterial Infections Endemic: Infection at persistently low level in specific geographical area. Epidemic: Higher than the usual. Pandemic: Spreads rapidly over large population area and is considered emergency which requires immediate remedial measures to control it. STAGES OF BACTERIAL PATHOGENESIS 1) 2) 3) 4) 5) Transmission from an external source into the portal of entry. Evasion of primary host defenses such as skin or stomach acid. Adherence to mucous membranes, usually by bacterial pili. Colonization by growth of the bacteria at the site of adherence. Disease symptoms caused by toxin production or invasion accompanied by inflammation. 6) Host responses, both nonspecific and specific (immunity), during steps 3, 4, and 5. 7) Progression or resolution of the disease. 29 Adherance Surface molecules on a pathogen, called adhesins or ligands, bind specifically to complementary surface receptors on cells of certain host tissues. Adhesins and ligands are usually on Fimbriae Examples: Neisseria gonorrhoeae, ETEC (Entertoxigenic E. coli), Bordetella pertussis , etc. Adhesin/ligand Cell surface Cell receptor 30 Capsule –prevent phagocytosis Examples: Streptococcus pneumoniae Klebsiella pneumoniae Haemophilus influenzae Bacillus anthracis Streptococcus mutans Yersinia pestis Klebsiella pneumoniae 31 Enzymes Many pathogens secrete enzymes that contribute to their pathogenicity A. Leukocidins - Attack certain types of WBC’s 1. Kills WBC’s which prevents phagocytosis 2. Releases & ruptures lysosomes that contain powerful hydrolytic enzymes which then cause more tissue damage B. Hemolysins - cause the lysis of RBC’s Streptococci 1. Alpha Hemolytic Streptococci - secrete hemolysins that cause the incomplete lysis or RBC’s 2. Beta Hemolytic Streptococci - Secrete hemolysins that cause the complete lysis of RBC’s Action of Hemolysins 32 Enzymes C. Coagulase - cause blood to coagulate Blood clots protect bacteria from phagocytosis from WBC’s and other host defenses. Staphylococci - are often coagulase positive Negative Positive 33 Enzymes D. Kinases - enzymes that dissolve blood clots 1. Streptokinase - Streptococci Streptokinase - used to dissolve blood clots in the Heart (Heart Attacks due to obstructed coronary blood vessels) 1. Staphylokinase - Staphylococci - Helps to spread bacteria causes Bacteriemia E. Hyaluronidase - Breaks down Hyaluronic acid (found in connective tissues) It is known as “Spreading Factor” Use of Hyaluronidase - Mixed with a drug to help spread the drug through a body tissue In cosmetics Hyaluronidase is used to dissolve the skin fillers. 34 Enzymes F. Collagenase -Breaks down collagen (found in many connective tissues) Clostridium perfringens - Gas Gangrene Uses - to spread through muscle tissue Used in cosmetics to fight cellulitis, contacture 35 Bacterial toxins Bacterial Toxins - Poisonous substances produced by microorganisms Toxins - primary factor of pathogenicity 220 known bacterial toxins 40% cause disease by damaging the Eukaryotic cell membrane Toxemia Toxins in the bloodstream 36 Toxins Toxigenesis: ability to produce toxins. Bacteria may produce two types of toxins: i. exotoxins and ii. endotoxins. Exotoxins are released from bacterial cells and may act at tissue sites remote from the site of bacterial growth. Endotoxins are cell-associated substance. (classic sense, endotoxin refers to the lipopolysaccharide component of the outer membrane of Gram-negative bacteria). 37 Toxins Endotoxins may be released from growing bacterial cells and cells that are lysed as a result of effective host defense (e.g. lysozyme) or the activities of certain antibiotics (e.g. penicillins and cephalosporins). Hence, bacterial toxins, both soluble and cell-associated, may be transported by blood and lymph and cause cytotoxic effects at tissue sites. Some bacterial toxins may also act at the site of colonization and play a role in invasion. 38 Toxins Cytotoxins – kills cells  Shigella  Vibrio Response to toxins If exposed to exotoxins: antibodies against the toxin (antitoxins ) Neurotoxins –interfere with normal nerve impulses  Cl. tetani  Cl.botulinum Exotoxins inactivated ( heat, formalin or phenol) no longer cause disease, but stimulate the production of antitoxin Enterotoxins – affect cells lining to the gastrointestinal tract  E.coli  Salmonella Altered exotoxins - Toxoids Toxoids - injected to stimulate the production of antitoxins and provide immunity 39 MECHANISMS OF PATHOGENICITY Portal of Entry Adherence Penetration/invasion of host defense Damage to host cell Important portals of entry 1. Mucus Membranes  Respiratory tract  Gastrointestinal tract  Genital 2. Skin 3. Parentarel Portals of entry of infection Ingestion (fecal-oral) Inhalation (respiratory) Trauma (e.g burn) Arthropod bite (zoonoses: mosquito, flea, tick, Tsetse fly) Sexual transmission Lathrogenic (needle stick, blood transfusion) Maternal-neonatal 42 Modes of infectious disease transmission Bacteria, fungi, virus… Ingestion: Salmonella, Shigella, Vibrio, Clostridium etc.. Inhalation: Mycobacterium, Mycoplasma, Chlamydia etc.. Trauma: Clostridium tetani Arthropod bite: Rickettsia, Yersinia pestis, etc. Sexual transmission: Neisseria gonorrhoeae, HIV, chlamydia, etc. Needle stick: Staphylococcus, HIV, HBV Maternal-neonatal: HIV, HBV, Neisseria 43 Common Diseases contracted via the Respiratory Tract Common cold Flu Tuberculosis Pneumonia Measles Diphtheria Common diseases contracted via the G.I. Tract Salmonellosis Salmonella sp. Shigellosis Shigella sp. Cholera Vibrio cholorea Ulcers Helicobacter pylori Botulism Clostridium botulinum Clostridium botulinum 3rd Portal of Entry: Parentarel Microorganisms are deposited into the tissues below the skin or mucus membranes Punctures and scratches injections bites surgery Transmission Means how the organism transmits the disease to the body cells: Human to human. Non human to human (fomites, animals, soil and water). Normal flora can become pathogenic at times but the infection is mostly acquired from the external sources. Modes of infectious disease transmission 48 Modes of infectious disease transmission Contact transmission Direct contact (person-to-person): syphilis, gonorrhearel herpes Indirect contact (formites): enterovirus infection, measles Droplet (less than 1 meter): whooping cough, strep throat Vehicle transmission Airborne: influenza, tuberculoses, chickenpox Water-borne (fecal-oral infection): cholera, diarrhea Food-borne: hepatitis, food poisoning, typhoid fever Vector transmission Biological vectors: malaria, plaque, yellow fever Mechanical vectors: E. coli diarrhea, salmonellosi 49 Transmission Bacterial diseases can be transmitted via food and insects. Animals can also act as a source (reservoir) or the mode of transmission (vector) of certain organisms. Diseases for which animals are the reservoirs are called zoonoses. Vertical transmission Bacteria, virus and other microbes can be transmitted via vertical transmission (mother to child). Across the placenta Birth canal Breast milk Transmitted via horizontal transmission (person to person). Extracellular versus Intracellular Parasitism Extracellular parasites - destroyed when phagocytosed. damaging tissues as they remain outside cells. inducing the production of opsonizing antibodies, they usually produce acute diseases of relatively short duration. Intracellular parasites can multiply within phagocytes. frequently cause chronic diseases. 52 Summary The Requirements for Growth Physical Requirements On the basis of temperature ranges, microbes are classified as psychrophiles (cold-loving), mesophiles (moderate temperature– loving), and thermophiles (heat-loving). Most bacteria grow best at a pH value between 6.5 and 7.5 In a hypertonic solution, most microbes undergo plasmolysis; halophiles can tolerate high salt concentrations 53 Summary The Requirements for Growth Chemical Requirements All organisms require a carbon source; chemoheterotrophs use an organic molecule, and autotrophs typically use carbon dioxide Nitrogen is needed for protein and nucleic acid synthesis. Nitrogen can be obtained from the decomposition of proteins On the basis of oxygen requirements, organisms are classified as obligate aerobes, facultative anaerobes, obligate anaerobes, aerotolerant anaerobes, and microaerophiles 54 Summary The Growth of Bacterial Cultures Bacterial Division The normal reproductive method of bacteria is binary fission, in which a single cell divides into two identical cells. The time required for a cell to divide or a population to double is known as the generation time. 55 Summary Phases of Growth During the lag phase, there is little or no change in the number of cells, but metabolic activity is high. During the log phase, the bacteria multiply at the fastest rate possible under the conditions provided. During the stationary phase, there is an equilibrium between cell division and death. During the death phase, the number of deaths exceeds the number of new cells formed. 56 Summary Methods of Microbial Control Heat Filtration Low Temperatures High Pressure Osmotic Pressure Radiation Disinfectants 57 Summary Pathogenicity The ability of a microbe to cause disease Virulence The degree of pathogenicity in a microorganism 58 References Gerard J. Tortora, Berdell R. Funke, Christine L. Case - Microbiology_ an introduction-Pearson (2018)