Characterizing and Classifying Prokaryotes and Viruses: Key Study Organisms

Gram-Positive Organisms (Phylum Firmicutes)

Spore-Forming Genera

Spore-forming bacteria are notable for their ability to survive extreme conditions by forming endospores. These genera are important in both environmental and clinical contexts.

• Clostridium: - General Characteristics: Anaerobic, rod-shaped, produces endospores. - Pathogenic Species: Clostridium botulinum (botulism), Clostridium tetani (tetanus), Clostridium difficile (colitis). - Exotoxins: Botulinum toxin, tetanospasmin. - Disease: Botulism, tetanus, pseudomembranous colitis.

• Bacillus: - General Characteristics: Aerobic or facultative anaerobic, rod-shaped, endospore-forming. - Pathogenic Species: Bacillus anthracis (anthrax). - Exotoxins: Anthrax toxin. - Disease: Anthrax.

Genus Lacking a Cell Wall

• Mycoplasma: - General Characteristics: No cell wall, pleomorphic, smallest free-living bacteria. - Pathogenic Species: Mycoplasma pneumoniae. - Disease: Atypical pneumonia ("walking pneumonia").

Pleomorphic Morphology

• Corynebacterium: - General Characteristics: Pleomorphic rods, often club-shaped. - Pathogenic Species: Corynebacterium diphtheriae. - Exotoxins: Diphtheria toxin. - Disease: Diphtheria.

Acid-Fast Bacilli

• Mycobacterium: - General Characteristics: Acid-fast due to mycolic acid in cell wall, slow-growing. - Pathogenic Species: Mycobacterium tuberculosis, Mycobacterium leprae. - Disease: Tuberculosis, leprosy.

Food-Spoilage Organisms

• Listeria: - General Characteristics: Facultative anaerobe, can grow at refrigeration temperatures. - Pathogenic Species: Listeria monocytogenes. - Disease: Listeriosis (foodborne illness).

Cocci Genera

• Staphylococcus: - General Characteristics: Gram-positive cocci in clusters. - Pathogenic Species: Staphylococcus aureus. - Exotoxins: Enterotoxins, toxic shock syndrome toxin. - Disease: Skin infections, food poisoning, toxic shock syndrome.

• Streptococcus: - General Characteristics: Gram-positive cocci in chains. - Pathogenic Species: Streptococcus pyogenes, Streptococcus pneumoniae. - Exotoxins: Streptolysins, erythrogenic toxin. - Disease: Strep throat, pneumonia, scarlet fever.

Gram-Negative Organisms (Phylum Proteobacteria)

Environmental and Pathogenic Genera

Gram-negative bacteria possess a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS). Many are environmental or pathogenic.

• Rhizobium: - General Characteristics: Nitrogen-fixing, symbiotic with plants. - Photosynthetic: No. - Disease: Not pathogenic.

• Rickettsia: - General Characteristics: Obligate intracellular parasites. - Pathogenic Species: Rickettsia rickettsii. - Disease: Rocky Mountain spotted fever.

• Brucella: - General Characteristics: Facultative intracellular pathogen. - Pathogenic Species: Brucella abortus. - Disease: Brucellosis.

• Neisseria: - General Characteristics: Diplococci. - Pathogenic Species: Neisseria gonorrhoeae, Neisseria meningitidis. - Disease: Gonorrhea, meningitis.

• Bordetella: - General Characteristics: Aerobic, coccobacilli. - Pathogenic Species: Bordetella pertussis. - Disease: Whooping cough (pertussis).

Facultative Anaerobic Opportunists

• Salmonella: - General Characteristics: Motile rods, facultative anaerobes. - Pathogenic Species: Salmonella enterica. - Disease: Salmonellosis, typhoid fever.

• Escherichia: - General Characteristics: Motile rods, facultative anaerobes. - Pathogenic Species: Escherichia coli. - Disease: Gastroenteritis, urinary tract infections.

Aerobic Opportunists

• Pseudomonas: - General Characteristics: Aerobic, motile rods. - Pathogenic Species: Pseudomonas aeruginosa. - Disease: Opportunistic infections, especially in immunocompromised patients.

Non-Motile, Facultative Anaerobes

• Pasteurella: - General Characteristics: Non-motile, coccobacilli. - Pathogenic Species: Pasteurella multocida. - Disease: Wound infections, zoonoses.

Helical Organisms

• Vibrio: - General Characteristics: Comma-shaped, motile. - Pathogenic Species: Vibrio cholerae. - Disease: Cholera.

• Helicobacter: - General Characteristics: Helical, motile. - Pathogenic Species: Helicobacter pylori. - Disease: Peptic ulcers, gastritis.

Genera of Sexually Transmitted Diseases (STDs)

• Treponema: - General Characteristics: Spirochete, motile. - Pathogenic Species: Treponema pallidum. - Disease: Syphilis.

• Chlamydia: - General Characteristics: Obligate intracellular, non-motile. - Pathogenic Species: Chlamydia trachomatis. - Disease: Chlamydia (STD), trachoma.

Obligate Anaerobes

• Bacteroides: - General Characteristics: Non-spore-forming, anaerobic rods. - Pathogenic Species: Bacteroides fragilis. - Disease: Intra-abdominal infections.

Viral Entities

Key Viruses Discussed

Viruses are acellular infectious agents, each with unique structure and disease associations. Below are important viruses for exam study.

• Herpes simplex viruses: - General Characteristics: Enveloped, double-stranded DNA. - Disease: Oral and genital herpes.

• Papillomavirus: - General Characteristics: Non-enveloped, double-stranded DNA. - Disease: Warts, cervical cancer.

• Retrovirus: - General Characteristics: Enveloped, single-stranded RNA, reverse transcriptase. - Disease: HIV/AIDS.

• Rhabdovirus: - General Characteristics: Enveloped, bullet-shaped, single-stranded RNA. - Disease: Rabies.

• Rubeolavirus: - General Characteristics: Enveloped, single-stranded RNA. - Disease: Measles.

• Variola virus: - General Characteristics: Enveloped, double-stranded DNA. - Disease: Smallpox.

• Varicella-Zoster virus: - General Characteristics: Enveloped, double-stranded DNA. - Disease: Chickenpox, shingles.

Summary Table: Key Bacterial Genera and Their Properties

Genus	Gram Stain	Cell Morphology	Pathogenic Species	Disease	Exotoxin
Clostridium	Positive	Rod, spore-forming	C. botulinum, C. tetani	Botulism, tetanus	Botulinum toxin, tetanospasmin
Bacillus	Positive	Rod, spore-forming	B. anthracis	Anthrax	Anthrax toxin
Mycoplasma	None	Pleomorphic	M. pneumoniae	Pneumonia	None
Corynebacterium	Positive	Pleomorphic rod	C. diphtheriae	Diphtheria	Diphtheria toxin
Mycobacterium	Acid-fast	Rod	M. tuberculosis	Tuberculosis	None
Listeria	Positive	Rod	L. monocytogenes	Listeriosis	None
Staphylococcus	Positive	Cocci (clusters)	S. aureus	Skin infections	Enterotoxins
Streptococcus	Positive	Cocci (chains)	S. pyogenes	Strep throat	Streptolysins
Neisseria	Negative	Diplococci	N. gonorrhoeae	Gonorrhea	None
Salmonella	Negative	Rod	S. enterica	Salmonellosis	None
Escherichia	Negative	Rod	E. coli	UTI, gastroenteritis	None
Pseudomonas	Negative	Rod	P. aeruginosa	Opportunistic infections	None
Treponema	Negative	Spirochete	T. pallidum	Syphilis	None
Chlamydia	Negative	Coccoid	C. trachomatis	Chlamydia	None
Bacteroides	Negative	Rod	B. fragilis	Abscesses	None

Additional info:

• Exotoxins are protein toxins secreted by bacteria, often responsible for disease symptoms.

• Gram-positive bacteria have thick peptidoglycan cell walls; Gram-negative bacteria have thin peptidoglycan and an outer membrane with LPS.

• Acid-fast staining is used for bacteria with waxy cell walls (e.g., Mycobacterium).

• Viruses are classified by genome type (DNA or RNA), envelope presence, and disease association.