MICROBIAL MECHANISM OF PATHOGENICITY 5262021 microbiology team 3

MICROBIAL MECHANISM OF PATHOGENICITY

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 OUT LINES q MICROBIAL GROWTH q MICROBIAL GENETICS q PATHOGENIC PROPERITIES OF MICROBES q CHAIN OF INFECTION 2

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 3 Objectives: • At the end of this lecture the students will be able to: ØUnderstand microbial growth ØDescribe microbial genetics ØDifferentiate between pathogenic properties of versus, fungi, helminthes and algae ØEnumerate the elements of infection cycle ØPredict & practice how infection cycle can be broken and infection can be prevented

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 4 Microbial Growth

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 5 1 - Microbial Growth It refers to an increase in cell number, not in cell size. There are basic requirements needed for bacterial growth which are both physical & chemical.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 6 A- Physical requirements of M. Growth § Temperature • On the basis of preferred temperature ranges , microbes are classified into : Psychrophiles (cold- loving microbes ) i. i. Example of disease caused by psychrophiles are dysentery, diarrhea, meningitis, sepsis, food poisoning, urinary tractinfections, and gastrointestinal infections. Mesophiles ii. i. (moderate –temperature loving microbes) Common types: staphylococcus aureus, salmonella and listeria. iii. Thermophiles (heat –loving microbes ) i. Can be the cause of meningitis, endocarditis, and septicemia.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 7 Temperature ……cont, . ØThe minimum growth temperature is the lowest temperature at which (microbes) will grow ØThe optimum growth temperature is the temperature at which microbes grows best. ØThe maximum growth temperature is the highest temperature at which growth is possible.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 8 A- Physical requirements §p. H Most bacteria prefer neutral p. H (6. 5 -7. 5). Molds and yeast grow in wider p. H range, but prefer p. H between 5 and 6. 4 Acidity inhibits most microbial growth and is used frequently for food preservation (e. g. : pickling). Alkalinity inhibits microbial growth, but not commonly used for food preservation.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 9 A- Physical requirements Organisms can be classified as: • A. Acidophiles : “Acid loving”. Grow at very low p. H (0. 1 to 5. 4) Lactobacillus produces lactic acid, tolerates mild acidity. • B. Neutrophiles : Grow at p. H 5. 4 to 8. 5. Includes most human pathogens. • C. Alkaliphiles : “Alkali loving”. Grow at alkaline or high p. H (7 to 12 or higher) Vibrio cholera

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 10 A- Physical requirement § Osmotic Pressure : Cells are formed of 80 to 90% water. A. Hypertonic solutions: High osmotic pressure removes water from cell, causing shrinkage of cell membrane (plasmolysis). Used to control spoilage and microbial growth. B. Hypotonic solutions: Low osmotic pressure causes water to enter the cell. In most cases cell wall prevents excessive entry of water. Microbe may lyse or burst if cell wall is weak

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 11 Isotonic , Hypotonic & Hypertonic Solution Plasmolysis

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 12

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 13 B- Chemical requirements of Mic. Growth • 1 - Carbon: Makes up 50% of dry weight of cell • 2 - Nitrogen, Sulfur, and Phosphorus: A. Nitrogen: Makes up 14% of dry cell weight. Used to form amino acids, DNA, and RNA B. Sulfur: Used to form proteins and some vitamins (thiamin and biotin). C. Phosphorus: Used to form DNA, RNA, ATP, and phospholipids. 3 - Other Elements: Potassium, magnesium, and calcium are often required as enzyme cofactors. 4. Trace Elements: . Many are used as enzyme cofactors. Commonly found in tap water. as Iron , Copper & Zinc

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 14 B- Chemical requirements of Mic. Growth • 5. Oxygen: • Organisms that use molecular oxygen (O 2 ), produce more energy from nutrients than anaerobes. We can classify microorganism based on their oxygen requirements to: • A. Obligate Aerobes: Require oxygen to live. Disadvantage : Oxygen dissolves poorly in water. Example: Pseudomonas, common nosocomial pathogen • B. Facultative Anaerobes: Can use oxygen, but can grow in its absence. Have complex set of enzymes. Examples: E. coli, Staphylococcus, yeasts, and many intestinal bacteria. • C. Obligate Anaerobes: Can not use oxygen and are harmed by the presence of oxygen. Examples: Clostridium bacteria that cause tetanus and botulism.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 15 B- Chemical requirements of Mic. Growth • D. Aerotolerant Anaerobes: • Can’t use oxygen, but tolerate its presence. Example: Lactobacillus carries out fermentation regardless of oxygen presence. • E. Microaerophiles: Require oxygen, but at low concentrations. . Example: Campylobacter.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 16 Culture Media • Culture Medium: Nutrient material prepared for microbial growth in the laboratory. Requirements: v Must be sterile v Contain appropriate nutrients v Must be incubated at appropriate temperature Culture: Microbes that grow and multiply in or on a culture medium

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 17 Bacterial Growth: Binary Fission

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Phases of Mic. Growth • Four phases of Bacterial Growth: • 1. Lag Phase: • Period of adjustment to new conditions. • Little or no cell division occurs, population size doesn’t increase. • Phase of intense metabolic activity, in which individual organisms grow in size. • May last from one hour to several days. 18

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 19 Phases of Mic. Growth • 2. Log Phase: • Cells begin to divide and generation time reaches a constant minimum. • Period of most rapid growth. Number of cells produced > Number of cells dying • Cells are at highest metabolic activity. • Cells are most susceptible to adverse environmental factors at this stage as radiation & antibiotics

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 20 Phases of Mic. Growth • 3. Stationary Phase: • Population size begins to stabilize. Number of cells produced = Number of cells dying • Overall cell number does not increase. • Cell division begins to slow down. • Factors that slow down microbial growth: • • Accumulation of toxic waste materials • • Acidic p. H of media • • Limited nutrients • • Insufficient oxygen supply

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 21 Phases of Mic. Growth • 4. Death or Decline Phase: • Population size begins to decrease. Number of cells dying > Number of cells produced • Cell number decreases at a logarithmic rate. • Cells lose their ability to divide. • A few cells may remain alive for a long period of time

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 22 Microbial Genetics

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 23 Microbial Genetics • Is the science of heredity; includes the study of genes, how they carry information, how they are replicated & how they are expressed

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 24 Related terminologies • Gene: it is the unit of heredity. It is a segment of DNA that carries, in its nucleotide sequence, information for specific biochemical or physiologic property. • Phenotype: All the heritable physical characters of the organism (Eye colour in humans, resistance to antibiotic in bacteria …. . etc. ) • Genotype: It means the information in the DNA that control the phenotype.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 25 Microbial Genetics • Structure and function of genetic material : 1 - DNA : Deoxyribonucleic acid contains the genetic instructions for the development and function of living things 2 - RNA: Ribonucleic acid 3 - Nucleotides composed of : ØPhosphate group ØPentose sugar ØNitrogenous base

5/26/2021 Genetic Materials microbiology team/ 3 rd level students 1439 -1440 26

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 27 Molecules of Genetics • The main molecules of genetics are called nucleic acids. • All the genetic information are stored as a sequence of bases through nucleic acids mainly in DNA and in RNA in some RNA viruses.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Structure of DNA • DNA (Deoxyribonucleic acid serves as organism’s genetic material. • It is divided into functional units (genes). • Most of DNA is double stranded. • The two strands held together by hydrogen bonds between A and T or G and C • It consists of non- identical, complementary base sequence 28

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Nucleotide The basic structure of DNA molecules is the Nucleotide 29

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Structure of RNA (Ribonucleic acid): • Structurally similar to DNA except: Most of RNAs are single stranded. Sugar is ribose instead of deoxyribose. Uracil base instead of thymine base 30

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 31 Differentiation between DNA & RNA functionally differ from DNA in which : • Some of RNAs are used as messenger molecules (m. RNA) to transfer information from DNA to protein • Some of RNA as a part of ribosomes (r. RNA ). • Some are adaptor molecules(t. RNA) Few RNA only acts as a genetic material like DNA

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 32 Function of DNA in Microbiology • I- Replication : which means that one DNA molecule gives two DNA molecules each one consists of one strand from the original DNA and the other strand is newly formed one • II- Gene expression (protein synthesis) : The mechanism by which the sequence of nucleotides in a gene determines the sequence of amino acids in a protein occurs in the following steps: • A. Transcription • B. Translation • C. post-translational processes

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 33 Transcription • The process by which a ss. RNA (single stranded RNA) is formed by RNA- polymerase using DNA as a template, this RNA is called m. RNA (messenger RNA).

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 34 PATHOGENIC PROPERITIES OF MICROBES

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 35 MICROBIAL PATHOGENICITY • Factors that Influence the degree of Pathogenicity and the Progression of Infection and Disease include: • 1 - Host factors: Age, sex, ethnicity, nutrition (diet), hormonal status; personal hygiene and immune status; Underlying disease or medical condition; Antibiotic or drug usage; Presence of foreign object (e. g. , splinter, catheter, sutures, etc. ); Innate differences between hosts • 2 - Microbial factors: Bacterial virulence factors; Inoculum size (dosage) External factors (e. g. , crowding; seasonal variations; hygiene, sanitation and public health; food processing, storage and preparation; etc.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 36 MICROBIAL PATHOGENICITY To cause disease a pathogen must: • Gain access to the host. Adhere to host tissues. • Penetrate or evade host defenses. • Damage the host, either directly or accumulation of microbial wastes

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 37 Pathogenic Properties of Virus • Viruses have mechanisms to evade host defenses. viruses grow inside host cells to hide from immune defense. • Kill immune cells e. g. HIV. • Cytopathic effects: - The visible effects of viral infection on host cell. Some effects will kill the cell and some will just change the cells. • Viruses stop DNA, RNA and/or protein synthesis e. g. Herpes virus block mitosis.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 38 Pathogenic Properties of Virus • Lysosomal autolysis of host cells e. g. Influenza: bronchiolar epithelium. • Production of inclusion bodies (visible viral parts inside the cell) can identify a particular virus e. g. Rabies virus: Negri bodies. • Syncytium formation (neighboring cells fuse together) e. g. Varicella Zoster virus. • Change in cell function e. g. Measles, production of interferon by host cell (triggers host immune response), induce antigenic changes on host cell surface (triggers destruction of infected cell by host immune response)

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 39 Pathogenic Properties of Fungi q. They produce toxins causing allergies or disease e. g. -chronic sinusitis. q. Stachybotrys: headaches, vomiting, mental disturbance. Invasive systemic mycosis in immune compromised patients e. g. Candida. q Mushrooms: mycotoxins may be hallucinogenic or deadly effect.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 40 Pathogenic Properties of Protozoa q They can grow inside host cells causing lysis e. g. Malaria (Plasmodium) q They use host cells as food source and produce wastes that cause disease.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 41 Pathogenic Properties of Algae • It produces neurotoxin substances e. g. shellfish poisoning

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Pathogenic Properties of Helminths 1 - Cellular damage evokes symptoms 2 - Use host tissue for their own growth 3 - Produce large parasitic masses 4 - Presence of parasite interferes with host function 5 - Parasite's metabolic waste can cause symptoms 42

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 43 CHAIN OF INFECTION

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 44 introduction • The standard model of infectious disease causation. • It has 3 corners (vertices). 1. Agent: A microbe that causes disease. 2. Host: An organism that harbors disease. 3. Environment: External factors that allows disease transmission.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 45

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 46 Dynamics of transmission Communicable diseases are transmitted from the reservoir/source of infection to susceptible host through. I. Source/ Reservoir II. Modes of Transmission III. Susceptible Host

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 47 I. Source or reservoir • These are natural habitat of infectious agents in which an infectious agent normally lives and multiplies. • Examples are; 1. Human reservoirs 2. Animal reservoirs (zoonotic diseases) 3. Reservoir in non-living things.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 48 1. Human reservoirs Cases • • • Primary case Index case Secondary cases According to spectrum of disease: • Clinical cases • (Mild/severe-typical/atypical) • Sub-clinical cases • Latent infection cases Type: • Incubatory • Convalescent • healthy • Duration: • • Temporary Chronic Carriers • Portal of exit: • • Urinary Intestinal Respiratory others

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 49 2. Animal reservoirs (zoonotic diseases) • Infection disease that are transmissible under natural conditions from vertebrate animals to man, e. g. rabies, plague, bovine tuberculosis etc.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 50 3. Reservoir in non-living things • Soil and inanimate matter can also act as reservoir of infection. E. g. soil may harbor agents that causes tetanus, anthrax etc.

II: MODES OF TRANSMISSION OF DISEASE Communicable diseases transmitted from the reservoir or source of infection to a susceptible host in many ways.

5/26/2021 52 microbiology team/ 3 rd level students 1439 -1440 Mode of transmission Direct contact Indirect transmission Vehicle-borne Vector-borne: Droplet infection Contact with soil Inoculation into skin or mucosa Trans-placental (vertical) Mechanical biological propagative Cyclo-prop. Air-borne Cyclo-develop. Fomite-born Unclean hands and fingers

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 1. Direct transmission: 1. Direct physical contact: From source/ reservoir to a susceptible host, e. g. skin to skin contact, STDs AIDS, eye infections etc. 2. Droplet infection: By saliva & nasopharyngeal secretions e. g. flu, diphtheria, whooping cough, tuberculosis, meningococcal meningitis etc. 53

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 54 3. Contact with soil: The disease agent is in soil e. g. hook worm infection, tetanus, mycosis etc. 4. Inoculation into skin or mucosa: The disease agent inoculated directly into skin or mucosa e. g. rabies, hepatitis B, etc. 5. Trans placental (vertical): e. g. Toxoplasmosis, others, rubella, cytomegalovirus, herpes simplex (TORCH)

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 55 2. Indirect transmission 1. Vehicle borne transmission: • The agents multiplies or develops in vehicle & transmits through agencies like water, vegetables, fruits, milk products, ice, blood, serum, tissue or organs transplantation. • E. g. diarrhea, typhoid, cholera, polio, hepatitis A, brucellosis, etc.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 56 2. Vector-borne transmission: Living carriers that transports infectious agents to a susceptible individual. It may be; i. Mechanical transmission: When a vector simply carries pathogenic microorganisms on their body and transfers them to food. E. g. flies and cockroaches.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 57 iii. Air borne: Transmission is carried in air in form of droplets and dust e. g. tuberculosis, influenza, chicken pox, measles, viruses, spores of fungi etc. iv. Fomite borne: These are inanimate substances other than water or food contaminated by the infectious discharge, e. g: diphtheria, typhoid bacillary dysentery, hepatitis A, eye and skin infections.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 58 iii. Susceptible host: • The person who is at risk for developing an infectious disease. • Factors making person susceptible are, i. age (young and elderly are more at risk) ii. underlying chronic diseases such as diabetes iii. Immuno suppression conditions like HIV chemotherapy, and malnutrition.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 59 Chain of Infection • Definition: Chain of infection is a model (a circle of links) used to understand the infection process. • Each circle represents a component in the cycle. • Each link must be present and in the sequential order for an infection to occur. • Breaking any link in the chain can disrupt the infection.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 60

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Different Ways to break Chain • Diseases have certain weak points • The basic approach in controlling the disease is to • Identify these weak points and • Break the weakest links in chain of transmission 61

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 62 Chain of Infection: • It is the process that begins when, 1. An infectious agent leaves its 2. Reservoir or host through a 3. Portal of exit and then 4. Is conveyed by some mode of transmission to 5. Enter through an appropriate portal of entry to 6. Infect a susceptible host

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 1. Infectious agent: • Infectious agent is the pathogen (germ) that causes diseases 63

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Breaking the Chain • Infectious agent • Early diagnoses and treatment • Cleaning, disinfection and sterilization • Infection prevention policies • Pest control 64

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 65 2. Reservoir or host • Principal habitat where infectious agents live and multiply • From there they may spread to cause infection. • Reservoirs may be humans, animals or environmental.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Breaking the Chain 2. Reservoir of infection Cleaning, disinfection and sterilization • Infection prevention policies • Pest control 66

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 67 3. Portal of exit: • It is the site from where micro-organisms leave the host to enter another host to cause infection. E. g. • Upper respiratory tract: saliva, sneeze, cough • Gastrointestinal tract: feces, vomit • Blood: infected blood • Uro - genital tract: semen, vaginal secretions, urine • Skin and mucous membranes: discharges

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 68 Breaking the Chain • 3. Portal of exit • Hand hygiene • Personal protective equipment • Control of aerosols and splatter (aerosols can transmit respiratory infections like tuberculosis, splashing of mucosa is also a potential risk) • Respiratory etiquette • Waste disposal

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 69 4. Modes of transmission • The route by which an infectious agent is transmitted from a reservoir to another host. • 6 Fs, food, fluid, fingers, fomites, feces, flies. • Modes of transmission are, • Direct transmission. Person to person: Touching, kissing, sexual intercourse, coughing, sneezing, talking etc ii. Trans – placental: Transfer of infectious agents from pregnant woman to fetus via placenta. i.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 70 • Indirect transmission. i. Air borne transmission: The infectious agent is present in air as dried secretions from respiratory tract, e. g. tuberculosis. ii. Vehicle borne transmission: any non-living substance that can be contaminated by an infectious agent, which then transmits it to a new host. iii. Vector borne transmission: An organism, usually an arthropod, which transmits an infectious agent to a new host. E. g. houseflies, mosquitoes, lice and ticks.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Breaking the Chain 4. Mode of transmission • Hand hygiene • Personal protective equipment • Food safety • Cleaning, disinfection, sterilization • Isolation 71

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 5. Portal of entry: • The site through which micro-organisms enter the susceptible host. • Inhalation: influenza • Ingestion: gastro enteritis • Needle prick: hepatitis B • Sexual contact: HIV/AIDS, • Open wound or punctures: tetanus 72

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Breaking the Chain 5. Portal of entry: • Hand hygiene • Personal protective equipment • Personal hygiene • First aid 73

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 74 6. Susceptible Host: • A organism that can be infected by pathogens. • Vulnerable Populations: Very young and the very old, ii. Immune suppressed (due to genetics, transplant drugs, malnutrition, or viral infection like HIV). iii. Occupational exposure. iv. Non-immune. i.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 Breaking the Chain 6. Susceptible Host: • Immunization • Treatment of under laying disease • Patient education 75

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 76 Patterns of Disease Introduction: • When the chain of infection is not broken, the infection and disease occurs. • A definite sequence of events usually occurs during and infection and disease. • Ultimately, the occurrence of disease depends on the resistance of the host to the activities of the pathogens. PREDISPOSING FACTORS • makes the body more susceptible to a disease and may alter the course of the disease. • Example, gender, genetics, geographic location, climate and weather,

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 77 Development of Disease • I. When microorganisms overcomes the defenses of the host, the disease develops following a certain sequence that are more like similar whether the disease is acute or chronic. INCUBATION PERIOD • This is the interval between the initial infection and the first appearance of any signs or symptoms. • The time of incubation depends on the specific microorganism involved such as: • virulence ( degree of pathogenicity), • the number of infecting microorganisms • Resistance to the host.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 78 II. PRODORMAL PERIOD • This is a relatively short period that follows a period of incubation. • This is characterized by early, mild symptoms of disease ( e. g. general aches and malaise) III. PERIOD OF ILLNESS • In this period, the disease is most severe. • The persons exhibits overt signs and symptoms of disease such as: • fever, chills, muscle pain (myalgia) • sensitivity to light ( photophobia) • sore throat ( pharyngitis) • lymph node enlargement (lymphadenopathy) • Gastrointestinal disturbances • During this period, the number of white blood cells (WBC) may increase or decrease • . If the disease is not successfully overcome ( successfully treated), the patient dies during this period.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 79 IV. PERIOD OF DECLINE • signs and symptoms subside • May take from less than 24 hours to several days. • During this period, the patient is vulnerable to secondary infections. V. PERIOD OF CONVALESCENCE • the person regains strength and the body returns to its prediseased state. • Recovery has occurred. • During this period also, the patient can spread the infection just like the incubation period.

5/26/2021 microbiology team/ 3 rd level students 1439 -1440 80