Survey of the Microbial World: Medically Important Bacteria

Domain Bacteria: Medically Important Bacterial Groups

Overview of Bacterial Classification

Bacteria are classified into various phyla, classes, and orders based on genetic, structural, and metabolic characteristics. This section surveys key Gram-negative and Gram-positive bacteria of medical importance, focusing on their taxonomy, diseases caused, transmission, and clinical features.

Gram-Negative Bacteria

Phylum Pseudomonadota: Class α-, β-, and γ-Proteobacteria

Proteobacteria are a major group of Gram-negative bacteria, many of which are pathogenic to humans. They are classified into several classes, including α-, β-, and γ-Proteobacteria, each containing important genera.

Table: Selected Gram-Negative Pathogens and Their Diseases

Bacterium	Disease	Reservoir	Transmission	Key Symptoms
Rickettsia prowazekii	Typhus (epidemic)	Humans, squirrels	Human louse	High fever, spotted rash, stupor
Rickettsia typhi	Typhus (endemic)	Rats	Rat fleas	Fever, rash, aches
Rickettsia rickettsii	Rocky Mountain Spotted Fever	Small mammals	Ticks	Fever, rash, headache
Bartonella henselae	Cat Scratch Fever	Cats	Cat scratch/bite	Fever, papule, lymphadenopathy
Bordetella pertussis	Whooping cough	Humans	Airborne	Severe coughing, gasping
Neisseria gonorrhoeae	Gonorrhea	Humans	Sexual contact	Painful urination, pus
Neisseria meningitidis	Meningococcal meningitis	Humans	Airborne	Headache, fever, stiff neck
Legionella pneumophila	Legionellosis	Warm water	Inhalation	Pneumonia, fluid in lungs
Vibrio cholerae	Cholera	Water, feces	Fecal-oral	Diarrhea, dehydration
Pseudomonas aeruginosa	Opportunistic infections	Environment	Contact	Cellulitis, pneumonia
Escherichia coli	Traveler’s diarrhea	Human gut	Fecal-oral	Diarrhea
Salmonella enterica	Salmonellosis, Typhoid fever	GI tracts	Fecal-oral	Diarrhea, fever
Shigella spp.	Bacillary dysentery	Human gut	Fecal-oral	Bloody diarrhea
Yersinia pestis	Bubonic plague	Mammals	Fleas, droplets	Buboes, necrosis
Haemophilus influenzae	Meningitis, pneumonia	Humans	Respiratory	Inflammation, cough
Helicobacter pylori	Peptic ulcers	Humans, mammals	Fecal-oral	Abdominal pain, nausea
Chlamydia trachomatis	Chlamydia, trachoma	Humans	Sexual, eye contact	Discharge, pelvic pain
Treponema pallidum	Syphilis	Humans	Sexual	Sores, rash
Borrelia burgdorferi	Lyme disease	Mammals, birds	Ticks	Bull’s-eye rash, joint pain

Transmission and Pathogenesis

• Fecal-Oral Route: Many enteric pathogens (e.g., E. coli, Salmonella, Shigella) are transmitted via ingestion of contaminated food or water. Proper hand hygiene and sanitation are critical for prevention.

• Vector-Borne Transmission: Diseases such as typhus, plague, and Lyme disease are transmitted by arthropod vectors (lice, fleas, ticks).

• Airborne and Droplet Transmission: Respiratory pathogens (e.g., Bordetella pertussis, Neisseria meningitidis, Haemophilus influenzae) spread via inhalation of droplets.

• Sexual Transmission: Neisseria gonorrhoeae, Chlamydia trachomatis, and Treponema pallidum are transmitted through sexual contact.

Examples and Clinical Features

• Salmonella enterica: Causes salmonellosis, characterized by diarrhea, fever, and abdominal cramps. Severe cases may involve bloody stools and systemic infection.

• Yersinia pestis: Responsible for bubonic plague, presenting with painful swollen lymph nodes (buboes), fever, and necrosis.

• Pseudomonas aeruginosa: An opportunistic pathogen causing infections in immunocompromised patients, especially in hospital settings.

• Helicobacter pylori: Associated with peptic ulcers and gastric cancer; infection is often asymptomatic but can cause abdominal pain and nausea.

• Chlamydia trachomatis: The most common bacterial STI and a leading cause of infectious blindness worldwide.

• Treponema pallidum: Causes syphilis, which progresses through stages with symptoms such as sores, rash, and neurological complications if untreated.

• Borrelia burgdorferi: Causes Lyme disease, notable for the characteristic bull’s-eye rash and potential for joint and neurological symptoms.

Gram-Positive Bacteria

Phylum Bacillota (Low G+C) and Actinobacteria (High G+C)

Gram-positive bacteria are divided into groups based on their genomic G+C content. Many are significant human pathogens, including spore-formers and toxin producers.

Table: Selected Gram-Positive Pathogens and Their Diseases

Bacterium	Disease	Reservoir	Transmission	Key Symptoms
Clostridium tetani	Tetanus	Soil	Puncture wounds	Muscle spasms, lockjaw
Clostridium botulinum	Botulism	Soil, food	Oral	Paralysis, respiratory failure
Clostridium perfringens	Gas gangrene	Soil	Wounds	Tissue necrosis, gas production
Clostridioides difficile	Antibiotic-associated diarrhea	Humans, pigs	Fecal-oral	Diarrhea, colitis
Bacillus anthracis	Anthrax	Soil	Contact, inhalation	Lesions, respiratory distress
Staphylococcus aureus	MRSA, skin infections	Humans, surfaces	Contact	Pus, swelling, fever
Streptococcus pyogenes	Strep throat, scarlet fever	Humans	Respiratory	Sore throat, fever
Streptococcus pneumoniae	Pneumonia	Humans	Airborne	Fever, cough, chest pain
Mycobacterium tuberculosis	Tuberculosis	Humans	Airborne	Cough, weight loss
Mycobacterium leprae	Leprosy	Humans, armadillos	Prolonged contact	Nerve damage, skin lesions
Corynebacterium diphtheriae	Diphtheria	Humans	Respiratory, skin	Fever, sore throat
Gardnerella vaginalis	Vaginosis	Humans	Contact	Discharge, odor

Examples and Clinical Features

• Clostridium tetani: Causes tetanus, characterized by painful muscle contractions and lockjaw.

• Clostridium perfringens: Responsible for gas gangrene, leading to tissue death and gas production in wounds.

• Bacillus anthracis: Causes anthrax, which can be cutaneous, inhalational, or gastrointestinal. Spores are highly resistant and can remain dormant in soil.

• Staphylococcus aureus: Notable for MRSA strains, which are highly antibiotic-resistant and cause pus-filled skin infections.

• Mycobacterium tuberculosis: Causes tuberculosis, a chronic lung infection with symptoms of cough, weight loss, and night sweats.

Transmission Pathways and Prevention

Fecal-Oral Transmission

Many gastrointestinal pathogens are transmitted via the fecal-oral route, often due to inadequate hand hygiene or contaminated food and water. Prevention includes proper sanitation, handwashing, and safe food handling.

Vector-Borne and Airborne Transmission

Vector-borne diseases are transmitted by insects or arthropods, while airborne diseases spread through respiratory droplets. Control measures include vector control, vaccination, and respiratory hygiene.

Summary Table: Key Features of Major Bacterial Pathogens

Pathogen	Gram Stain	Reservoir	Transmission	Prevention
Salmonella enterica	Negative	GI tracts	Fecal-oral	Handwashing, food safety
Yersinia pestis	Negative	Mammals	Fleas, droplets	Vector control
Clostridium tetani	Positive	Soil	Puncture wounds	DTaP vaccine
Staphylococcus aureus	Positive	Humans	Contact	Hygiene, wound care
Mycobacterium tuberculosis	Positive (acid-fast)	Humans	Airborne	BCG vaccine, isolation

Conclusion

This survey highlights the diversity of medically important bacteria, their classification, and the diseases they cause. Understanding their transmission, reservoirs, and clinical features is essential for diagnosis, treatment, and prevention in clinical microbiology.