Micro_Chapter_11

Objectives of the Chapter

• Describe the features of each phylum, class, order, genus, and species of bacteria and archaea with specific examples and their relevance.

• Identify major characteristics of different gram-negative and gram-positive bacteria, including their structural differences, roles in human health and disease, and clinical implications.

• Recognize common reservoirs (environmental, animal, and human) and transmission routes (fecal-oral, vector-borne, direct contact) of various bacteria to understand infection dynamics.

• Identify the clinical significance of key bacterial species, discussing their pathogenic mechanisms, symptoms they cause, and their implications for public health.

• Outline key characteristics of domain archaea, including their unique metabolic pathways, ecological roles, and adaptations to extreme environments.

Overview of Prokaryotes

• Prokaryotes are organisms that lack a membrane-bound nucleus. They are primarily categorized into two domains:

  • Bacteria: A diverse group of microorganisms that are ubiquitous in nature and exhibit a wide range of metabolic capabilities.

  • Archaea: A distinct group of prokaryotes often found in extreme environments (extremophiles) and have unique biochemical properties.

• Each domain is divided into various taxonomic ranks including phylum, class, order, family, genus, and species, allowing for systematic classification of prokaryotic life.

• A crucial aspect in this chapter is the differentiation between gram-negative and gram-positive bacteria, particularly examining their cell wall compositions, staining characteristics, and implications for antibiotic treatment.

Domain Bacteria

Gram-Negative Bacteria

Pseudomonadota Phylum

• This phylum consists of gram-negative, chemoheterotrophic bacteria characterized by a unique rRNA sequence and a complex cell envelope that includes a thin peptidoglycan layer sandwiched between two membranes.

• It is the largest and most diverse group of bacteria, playing vital roles both ecologically and clinically.

• Divided into five classes:

  • Alpha Proteobacteria

  • Beta Proteobacteria

  • Gamma Proteobacteria

  • Delta Proteobacteria

  • Campylobacterota

Alpha Proteobacteria Class

• Pelagibacter ubique: Recognized as one of the most abundant microorganisms in the oceans, it plays a key role in marine carbon cycling and is non-pathogenic.

• Acetobacteraceae Family: Significant genera include Acetobacter and Gluconobacter, which participate in the fermentation process and convert ethanol into acetic acid, essential for vinegar production.

• Granulobacter: An emerging pathogen linked to chronic granulomatous disease, affecting immune regulation and posing risks to individuals with compromised immunity.

• Rickettsia Genus: These obligately intracellular bacteria are transmitted by insect and tick bites and can cause serious diseases:

  • Spotted Fever Rickettsiosis any infections cause febrile illness with rash.

  • Epidemic Typhus caused by R. prowazekii, transmitted through lice, significant in historical pandemics.

  • Endemic Typhus caused by R. typhi, transmitted by rat fleas, leading to febrile illness.

  • Rocky Mountain Spotted Fever caused by R. rickettsii, transmitted by ticks, characterized by rash and systemic involvement.

• Ehrlichia Genus: A group of intracellular pathogens affecting white blood cells, associated with tick-borne illnesses and can lead to severe febrile illnesses.

• Rhizobium & Bradyrhizobium: These genera form symbiotic relationships with legumes, enriching soil fertility through nitrogen fixation, critical for sustainable agriculture.

• Bartonella Genus: Bartonella henselae, the causative agent of cat scratch fever, transmitted via scratches or bites from infected cats, presenting as lymphadenopathy.

• Brucella Genus: Known for causing brucellosis, transmitted through contact with infected animals or unpasteurized dairy products; presents with fever, sweats, and malaise.

• Nitrobacter & Nitrosomonas: Important nitrifying bacteria involved in the nitrogen cycle, crucial for plant nutrition and soil health.

Beta Proteobacteria Class

• Spirulina Genus: Found in freshwater environments, motile cyanobacteria often used as a dietary supplement due to their high protein content and nutrient density.

• Burkholderia Genus: Particularly notable is Burkholderia cepacia, which poses a significant risk to cystic fibrosis patients, often leading to severe respiratory infections.

• Bordetella Genus: Includes Bordetella pertussis, responsible for whooping cough, highly contagious and preventable through vaccinations, known for its severe respiratory symptoms.

• Neisseria Genus: This includes major human pathogens like Neisseria gonorrhoeae (gonorrhea) and Neisseria meningitidis (meningococcal meningitis), both of which can lead to severe health complications and require prompt treatment.

Gamma Proteobacteria Class

• Francisella Genus: Includes pathogens such as Francisella tularensis, which causes tularemia; transmission occurs via insect bites, contaminated water, or direct contact with infected animals.

• Pseudomonas Genus: Pseudomonas aeruginosa is an opportunistic pathogen known for causing infections in immunocompromised individuals, particularly in healthcare settings; associated with pneumonia and urinary tract infections.

• Moraxella Genus: Moraxella lacunata is primarily linked to conjunctivitis, particularly in children, and can also be implicated in upper respiratory infections.

• Acinetobacter Genus: This genus includes opportunistic pathogens that cause various healthcare-associated infections and exhibit significant antibiotic resistance, particularly Acinetobacter baumannii.

• Legionella Genus: Associated with legionellosis, caused by Legionella pneumophila, which thrives in aquatic environments and can lead to severe pneumonia when inhaled in aerosolized forms.

• Coxiella Genus: Coxiella burnetii causes Q fever, often transmitted through inhalation of contaminated aerosols or ingestion of unpasteurized dairy products.

• Vibrio Genus: Includes important pathogens such as Vibrio cholerae (cholera) and Vibrio parahaemolyticus (gastroenteritis), both associated with contaminated water and undercooked seafood.

• Enterobacteriaceae Order: A diverse group of bacteria with genera critical for gastrointestinal health and associated with various diseases. Important genera include:

  • Escherichia Genus: E. coli, a normal gut inhabitant; however, particular strains like E. coli O157:H7 are pathogenic and can cause severe gastrointestinal disease.

  • Salmonella Genus: Includes Salmonella enterica, responsible for salmonellosis, and Salmonella typhi, linked to typhoid fever, emphasizing the importance of proper food handling and sanitation.

  • Shigella Genus: Responsible for shigellosis, a highly contagious diarrheal disease mainly spread through fecal-oral transmission.

  • Klebsiella Genus: Klebsiella pneumoniae is known for causing pneumonia, especially in hospitalized patients, as well as urinary tract infections.

  • Serratia Genus: Serratia marcescens can cause infections in various sites including lungs, urinary tract, and bloodstream, often in immunocompromised patients.

  • Proteus Genus: Frequently implicated in urinary tract infections, known for its urease production which raises urinary pH.

  • Yersinia Genus: Yersinia pestis is historically significant, known for causing bubonic plague through flea bites or contact with infected materials; Yersinia enterocolitica can cause gastrointestinal diseases.

• Enterobacter Genus: Associated with numerous healthcare-related infections, particularly in the urinary and respiratory tracts.

Pasteurellales Order

• Pasteurella Genus: Causes infections in animals which can be transmitted to humans, typically through bites from domestic pets, with Pasteurella multocida causing wound infections.

• Haemophilus Genus: Comprises pathogens like Haemophilus influenzae, significant for causing bacterial meningitis, respiratory infections, and requiring specific growth factors for culture.

Campylobacterota Class

• Campylobacter Genus: Campylobacter jejuni is a leading cause of foodborne intestinal disease, often associated with undercooked poultry and contaminated water supplies.

• Helicobacter Genus: Helicobacter pylori is a notable pathogen linked to chronic gastritis and peptic ulcers, demonstrating unique ability to survive in acidic environments through urease production.

Chlamydia Phylum

• Chlamydia Genus: Distinct for lacking a peptidoglycan cell wall, these pathogens cause various infections:

  • Chlamydia trachomatis: leading cause of sexually transmitted infections and can cause infertility.

  • Chlamydia psittaci: transmissible from birds to humans, causing psittacosis.

  • Chlamydia pneumoniae: associated with respiratory tract infections, including mild pneumonia.

Bacteroidota Phylum

• Includes genera such as Bacteroides, essential in the human gut microbiome, aiding digestion and metabolism; some species can lead to infections if they move from the gut to other body sites.

Spherochaetota Phylum

• Treponema Genus: Includes pathogenic species like Treponema pallidum (syphilis); Borrelia species are responsible for Lyme disease and other relapsing fevers, with transmission often through tick bites.

• Leptospira Genus: Known for causing leptospirosis, transmitted in contaminated water; presents with flu-like symptoms and can lead to more serious complications.

Gram-Positive Bacteria

Bacillota Phylum

• Comprises several important genera with varied clinical significance:

  • Clostridium: Pathogenic representatives include:

    • Clostridium tetani: produces tetanospasmin, leading to muscle spasms and lockjaw.

    • Clostridioides difficile: associated with antibiotic-associated diarrhea and colitis, often treated with specific antibiotics or fecal transplants.

    • Bacillus anthracis: causes anthrax, a serious infection that can take cutaneous, inhalational, or gastrointestinal forms, necessitating stringent biosafety practices.

• Lactobacillales Order:

  • Lactobacillus: Important for fermentation processes, impacting gut health and food production through lactic acid production.

  • Staphylococcus: Especially Staphylococcus aureus, a major cause of skin infections, pneumonia, and foodborne illnesses, known for developing antibiotic resistance (e.g., MRSA).

  • Streptococcus: Contains many pathogens that produce various virulence factors and can cause diseases ranging from mild pharyngitis to severe invasive infections.

Mycoplasmatota Phylum

• Characterized by organisms such as Mycoplasma, which lack cell walls and can cause atypical pneumonia; their unique biology makes them resistant to many typical antibiotics.

Actinomycetota Phylum

• Mycobacterium: Contains species of significant medical importance:

  • Mycobacterium tuberculosis: known to cause tuberculosis, a serious airborne disease requiring multi-drug therapy for treatment.

  • Mycobacterium leprae: causes leprosy, impacting skin and peripheral nerves, necessitating long-term antibiotic treatment.

Domain Archaea

• Currently, there are no known pathogenic archaea; most are extremophiles adapted to harsh environments.

• Four nutritional groups:

  • Halophiles: Thrive in high-salt environments (e.g., salt lakes); involve in various biochemical processes.

  • Thermophiles: Prefer hot habitats (e.g., hot springs, hydrothermal vents), posses unique enzymes useful in biotechnology.

  • Acidophiles: Flourish in acidic environments, significant in biogeochemical cycles.

  • Methanogens: Produce methane, are important in digestive processes in some animals and humans, contributing to the natural biogeochemical cycling of carbon.