Microbiology: Bacteria Classification & Structure

• Infection occurs when a microbe or parasite reproduces in or on the body's tissues, causing disease.
• Microorganisms are classified into pathogenic and nonpathogenic.
• Virulence and pathogenicity levels differ among microorganisms.
• Louis Pasteur proposed the germ theory in the 19th century.

Types of Microorganisms

• Bacteria
• Virus
• Fungi
• Protozoa
• Helminth

Bacteria

• Bacteria are unicellular organisms that do not require living tissues to survive.
• They vary in size and shape and are classified accordingly.
• Bacteria reproduce through binary fission.

Classification of Bacteria

• Bacteria are classified based on shape: bacillus (rod), coccus (sphere), and spirillus (spiral).
• Bacteria are classified based on cell wall structure: gram-positive and gram-negative.
• Bacteria are classified based on growth response: aerobes (e.g., Bordetella pertussis) and anaerobes (e.g., Clostridia).

Structure of Bacteria

• The height of a bacterium structure is 3-4 μm.
• Flagellum confers motility.
• Pili adhere to host cell surface structures.
• The capsule is a polysaccharide structure that may be shed or dissolve.
• Some cells contain circular DNA plasmids coding for toxin production or antibiotic resistance.

Common Bacteria and Infections

• Staphylococcus aureus causes skin infections, pneumonia, sepsis, and endocarditis.
• Clostridium difficile causes diarrhea and pseudomembranous colitis.
• Klebsiella pneumoniae causes pneumonia.
• Escherichia coli causes diarrhea, urinary tract infections, respiratory disease, and sepsis.
• Pseudomonas aeruginosa causes sepsis, wound, and burn infections.
• Bacteroides fragilis causes gastrointestinal upset.
• Mycobacterium tuberculosis causes tuberculosis.

Mechanism of Bacterial Infection

• The mechanism depends on the bacteria's structure.
• Bacteria secrete toxic substances like toxins and enzymes.
• Toxins consist of exotoxins and endotoxins.

Exotoxins

• Exotoxins are usually produced by gram-positive bacteria.
• They are highly toxic.
• They pass out through the cell membrane and diffuse into the surrounding medium.
• Exotoxins slowly deteriorate at room temperature.
• They are highly antigenic.
• They are neutralized by antibodies.
• They are highly specific for certain tissues or organs.
• They are very active even in small doses.
• Exotoxins are heat labile and destroyed at 65°C.

Endotoxins

• Endotoxins are produced chiefly by gram-negative bacteria.
• They are weakly toxic.
• They remain enclosed within the cell membrane.
• There is no deterioration at room temperature.
• They are weakly antigenic.
• Neutralizing antibodies are not formed.
• They are not specific towards tissues or organs.
• Very large doses are required.
• Endotoxins are heat stable and not destroyed even at 100°C.

Viruses

• Viruses are very small obligate intracellular parasites requiring a living host cell for replication.
• Viruses outside a host are called virions.
• Virions have a protein coat (capsid) and either DNA or RNA.
• Viruses undergo mutation.
• Viruses are of different shapes.

Common Viral Diseases

• Orthomyxoviruses (RNA) cause influenza A, B, and C.
• Paramyxoviruses (RNA) cause mumps and measles.
• Togavirus (RNA) causes rubella (German measles) and hepatitis C.
• Herpesvirus (DNA) causes herpes simplex and varicella (chickenpox).
• Flaviviruses (RNA) cause West Nile virus.
• Picornaviruses (RNA) cause poliovirus, hepatitis A, and encephalitis.
• Hepadnaviruses (DNA) cause hepatitis B virus.
• Papovaviruses (DNA) cause warts and cancer (human papillomavirus/HPV).
• Retrovirus (RNA) causes HIV.

Mechanism of Viral Infection

• A virus attaches to a host cell and penetrates it.
• The viral DNA or RNA enters the host cell nucleus and takes control of the host cell DNA.
• The host cell synthesizes viral components.
• New viruses assemble.
• Many new viruses and host cell lysis are released.

Fungi

• Most fungi are multicellular organisms.
• However, not all fungi are considered to be microorganisms.
• Fungi reproduce by means of spores.
• Fungal or mycotic infection stems from single-celled yeasts or multicellular molds.
• These organisms are classified as eukaryotic.
• They consist of single cells or chains of cells, which can form a variety of structures.

Common Fungal Infections

• Candida albicans causes thrush.
• Aspergillus fumigatus causes lung infection in immunocompromised patients.
• Trichophyton interdigitale causes athlete's foot.
• Pneumocystis jirovecii causes pneumonia in immunocompromised patients.

Mechanism of Fungal Infection

• Diseases associated with fungal infections are mostly seen in those with compromised immune systems.
• Many fungi cause opportunistic infections.

Protozoa

• Protozoa are single-cell eukaryotes.
• They are unicellular, motile, and lack a cell wall, but occur in several shapes, sometimes within the life cycle of a single cell.
• Protozoan pathogens are usually parasitic.

Common Diseases Caused by Protozoa

• Giardia lamblia causes giardiasis.
• Toxoplasma gondii causes toxoplasmosis.
• Entamoeba histolytica causes amoebic dysentery.
• Plasmodium vivax causes malaria.

Mechanism of Protozoan Infection

• Clinically, protozoa are found in RBCs and undergo several stages in their life cycle.
• The RBCs become large and eventually rupture, releasing new microbes and toxins into the blood, causing acute illness.
• Microbes are transmitted by blood-sucking insects, such as the female Anopheles mosquito.

Helminths

• Helminths, or worms, are not microorganisms.
• They are grouped with microbes due to their role as parasites causing human infections.
• Helminths are multicellular, eukaryotic organisms that vary in size.
• Their life cycle includes three stages: ovum (egg), larva, and adult.
• Helminths can enter the body through contaminated food, water, skin, or infected insects.
• They usually inhabit the intestine but may be found in the lungs or blood vessels at different life stages.
• Ectoparasites include insects (lice, fleas) and arachnids (mites, ticks, spiders).

Mechanism of Helminth Infection

• Adult tapeworms lay eggs in the host intestine, which are passed in feces.
• Intermediate hosts ingest these eggs.
• The eggs hatch into larvae, which then enter the bloodstream, forming cysts in organs.
• Humans consume raw or undercooked meat and become infected.

Pathophysiology of Infection

• Pathophysiology of infection refers to biological and physiological changes in the body when a pathogen (bacteria, virus, fungi, or parasite) invades, multiplies, and causes disease.
• It encompasses the interplay between the pathogen's virulence factors and the host's immune response.

Pathophysiology of Infection: Steps

• Pathogens enter the body through various routes: respiratory tract, gastrointestinal tract, skin or mucosa, genital tract, and blood.
• Pathogens use specialized structures such as (adhesins, pili, or surface proteins) to bind to host cell receptors.
• Pathogens establish themselves at the site of infection by multiplying and competing with normal flora for resources. They use host nutrients and machinery (for viruses): colonization and replication.
• Some pathogens invade deeper tissues and spread systemically: invasion and spread.
• Pathogens use strategies to avoid detection and destruction by the immune system: capsules prevent phagocytosis (e.g., Streptococcus pneumoniae).
• Pathogens cause tissue damage through direct and indirect effects.
• Innate immune response: Activation of macrophages, neutrophils, and complement proteins limit pathogen spread.
• Release of inflammatory cytokines (IL-1, TNF-α) causes fever, swelling, and pain.
• Adaptive immune response: Activation of T cells and B cells, production of antibodies to neutralize it and cytotoxic T cells kill infected host cells.
• Fever, fatigue and malaise, and organ dysfunction occur.
• The infection is cleared by the immune system, often with medical interventions like antibiotics or antivirals.

Acute Disease Model of Tetanus

• Tetanus is a nervous system infection caused by Clostridium tetani, which is potentially deadly if untreated.

Risks Factors for Tetanus

• Tetanus can affect individuals of all ages, but newborns and older adults are at higher risk. Neonates are vulnerable to neonatal tetanus from unclean birth practices. Older adults may have weaker immune responses to the bacteria.
• Tetanus is preventable through vaccination. Unvaccinated individuals, or those who haven't received a recent booster, are most at risk.
• Lack of vitamins C, D, and E can weaken the immune system, increasing the risk of infection. Deficiencies in vitamin A, zinc, or iron can impair immune function, making it harder for the body to control bacterial infections.
• Exposure to pollutants like heavy metals, pesticides, and air contaminants can weaken the immune system, increasing susceptibility to contract an infection.
• People are more likely to recover with preventive care. A lack of healthcare access and health promotion can lead to a higher mortality rate.
• Clostridium tetani grows best in warm humid climate.

Exposure and Infection

• Tetanus is contracted when spores from Clostridium tetani enter the body through puncture wounds, cuts, burns, or breaks in the skin.
• Wounds often provide a low-oxygen environment ideal for bacteria to grow and produce tetanospasmin.
• The tetanospasmin interferes with the nerves that control muscles, causing muscle stiffness and spasms, usually starting with the jaw (lockjaw) and spreading.

Virulence Factors

• Tetanolysin damages the tissue surrounding the infection and optimizes the conditions for bacterial multiplication.
• Once Clostridium tetani enters the tissue, it releases tetanospasmin, which enters capillaries and travels to the nervous system, causing symptoms.

Mechanism of Tetanus

• Tetanospasmin binds to nerve endings in the bloodstream.
• It blocks the release of neurotransmitters like glycine and gamma-aminobutyric acid (GABA), which inhibit muscle contraction.
• Without these inhibitory signals, the muscles become overstimulated, causing uncontrolled contractions, leading to muscle spasms.

Incubation Period

• The incubation period for tetanus is typically between 3 to 21 days.
• Most cases manifest symptoms around 7-10 days after the wound becomes infected.
• Signs and symptoms worsen over two weeks, starting at the jaw.
• Tetanus symptoms: painful muscle spasms, lip tension, painful spasms and rigidity, difficulty swallowing, and rigid abdominal muscles.

Illness Phase

• As tetanus worsens, people have frequent, excruciating, seizure-like spasms (generalized spasms).
• The arms are dragged up to the body, the fists are clinched, and the knees become rigid.
• Breathing issues may be brought on by stiff neck and abdominal muscles.
• As the disease progresses, signs and symptoms include hypertension, hypotension, tachycardia, fever, extreme sweating, and respiratory distress.
• Toxin affects autonomic functions, causing blood pressure fluctuations, arrhythmias, fever, and excessive sweating.
• Spasms of the diaphragm and chest muscles can impair breathing and lead to respiratory failure.

Tetanus Treatment

• Tetanus vaccine stimulates the body to produce antibodies that neutralize toxins.
• There is no separate vaccine just for tetanus, but a combination known as the diphtheria, tetanus, pertussis vaccine (DPT or pentavalent vaccine).
• DPT vaccine is given intramuscularly at 2 months, 4 months, 6 months, 15-18 months, and 4-6 years.
• Pregnant women receive the Tdap vaccine between 27 and 36 weeks of pregnancy.
• After the initial series, boosters (Tdap vaccine) are recommended every 10 years by the CDC.
• Tetanus immune globulin (TIG) is also recommended.

Disease Model of Chronic Viral Hepatitis B

• Hepatitis B virus is a DNA virus transmitted through blood-to-blood contact, unprotected sex, perinatal transmission, and unsterile medical and dental equipment.
• The virus infects hepatocytes and starts replicating.
• The immune system responds by clearing the virus, which results in liver damage.
• If the virus is not removed, it can cause complications like fibrosis or cirrhosis.
• Acute HBV usually lasts for 4-12 weeks. Symptoms range from no symptoms to liver failure.
• Chronic HBV occurs if the immune system fails to destroy all the HBV and remains in the body for more than 6 months, leading to an increased risk for liver cancer.
• Most individuals with HBV are asymptomatic and may remain asymptomatic carriers.
• Symptomatic phase includes jaundice, anorexia, nausea, edema, fever, and stomach pain.
• Preventing Hepatitis B: Vaccination, screening, proper hygiene, and safe sex.
• Risk factors for Hepatitis B include unvaccinated individuals, high-risk behaviors, traveling to regions with high infection rates, a weakened immune system, and perinatal exposure.
• Treatment of acute HBV involves supportive care, rest, increased fluid intake, and anti-viral drugs (e.g., Tenofovir).
• Treating chronic HBV involves antiviral medications to suppress viral replication, continuous monitoring of the liver, and regular monitoring of HCC.
• Most cases of acute HBV are normally resolved with immunity.
• Chronic infection leads to liver disease (cirrhosis)
• The outcome varies depending on age, viral load, immune response, and treatment.

Physiology of Treatment for Hepatitis B

• Hepatitis B causes inflammation of the liver caused by the hepatitis B virus.
• Treatment involves using antivirals and interferon therapy.
• The mechanism of antiviral medication increases cell resistance to viruses, suppress virus adsorption, and diffusion, and inhibits nucleic acid synthesis.
• Antiviral treatment side effects include fever, nausea and vomiting, skin rash, headache, diarrhea, dry mouth, and fatigue. Antiviral medications for Hepatitis B are Tenofovir and Entecavir.
• Interferon therapy boosts the immune system against chronic Hepatitis B by producing T and macrophage cells that clear infected cells and control viral replication.
• Side effects of interferon therapy: chills, fever, trouble sleeping, Nausea, vomiting, diarrhea, irritability, muscle pain, low levels of white blood cells, loss of appetite, and itchy skin.

Treating Bacterial Infections

• Salmonellosis: A common bacterial disease affecting the gastrointestinal tract.
• Salmonella bacteria typically live in animal and human intestines and are shed through feces.
• Antibiotics Inhibit bacterial adhesion, cell wall, protein and nucleic acid synthesis, folate synthesis and target specific enzymes.
• Effects of antibiotics are nausea, indigestion, vomiting, diarrhea, bloating or filling, stomach pain and cramping, and loss of appetite.
• Ampicillin, Ceftriaxone, and Ciprofloxacin are antibiotic used to treat salmonellosis.

Oral Candidiasis Treatment

• Oral candidiasis is also called oral thrush, and is a condition where the fungus Candida albicans builds up in the mouth.
• Antifungal medication is used by disrupting the cell membrane, where without the presence of ergosterol cells membranes become unstable leading to cell damage.
• They can also hinder fungal cell wall synthesis.
• By targeting B-glucan major component in fungal cells wall, antifungals make the cells susceptible to osmotic stress causing cell lysis.
• Antifungal medicines also stops nucleic acid synthesis and cell mitosis.
• Side effects of antifungal medication: vomiting, diarrhea, nausea, muscle and joint pain and stomach ache.
• Example medication: Nystatin suspension and Fluconazole capsules are prescribed for treating Oral candidiasis.

Parasitic Infection

• Trichomoniasis is a sexually transmitted infection caused by a tiny single-celled parasite.
• Anthelmintics inhibits tubulin and interrupts microtubules formation which stops cells division, nutritions, absorption and motility.
• Antiprotozoal interacts with DNA and causes breakages which stops cell replications.
• GI discomfort, fever, dizziness, rashes, headaches and nausea are side effects of antiparasitic medication.
• Examples of Medications: Ivermectin is use to treat larvae of worms, Mebendazole used to treat round worm, hookworm, pinworm and whipworm, and Tinidazole used damage DNA of parasites.