Pathogenic Bacteria: Staphylococci and Streptococci

Pathogenic Bacteria

• Staphylococci are gram-positive cocci found widely in nature
• Three medically important species are Staph. aureus, Staph. epidermidis, and Staph. saprophyticus
  • Staph. aureus is the most important, differentiated by being coagulase-positive and mannitol-positive, and causing blood agar hemolysis
• They are considered potentially pathogenic
• All Staphylococci are catalase-positive, unlike Streptococci which are catalase-negative
• Staphylococci produce intracellular (endo) pigments which contribute to coloration
• Staphylococci cause acute toxic food poisoning and toxic shock syndrome

Infections Caused by Staphylococci

• Staph. epidermidis: non-pigmented, non-hemolytic colonies, commonly hospital-acquired
• Staph. saprophyticus: colonies are white or yellow, non-hemolytic, urinary tract infections.

Streptococci

• Gram-positive cocci arranged in chains, of variable length; widely distributed and non-spore-forming
• An important characteristic for identifying streptococci is the type of hemolysis they produce on blood agar
• Beta-hemolytic streptococci produce complete haemolysis with a clear zone. An example is Strepto. pyogenes.
• Alpha-hemolytic streptococci produce greenish discoloration due to the production of H2O2 from the breakdown of haemoglobin. Examples include Streptococci viridans and Strepto. pneumoniae.

Diseases Caused by Streptococci

• Strept. pyogenes: sore throat, scarlet fever, toxic shock syndrome
• Strept. viridans: part of the normal mouth and throat flora; few species are pathogenic and can be implicated in dental caries, bacteremia, and subacute bacterial endocarditis

Streptococcus Pneumoniae (Pneumococci)

• Members of the normal upper respiratory tract flora
• Causes lobar pneumonia and its complications (e.g., sinusitis, meningitis)
• Gram-positive cocci arranged in pairs
• Alpha-hemolysis, or greenish discoloration, similar to Strept. viridans
• Quellung test

Neisseria

• Found in the mouth, throat, pharynx, and vagina
• Two important pathogens are N. meningitidis and N. gonorrhoeae
• Gram-negative cocci arranged in pairs, oxidase-positive
• N. gonorrhoeae and N. meningitidis require enriched media for growth
• N. meningitidis: causes acute epidemic cerebrospinal meningitis
• N. gonorrhoeae: causes gonorrhoea in adult males and vulvo-vaginitis in young girls; a venereal disease

Corynebacterium

• C. diphtheriae produces a powerful exotoxin causing diphtheria in humans
• Gram-positive rods
• Occur as normal flora on the skin and mucous membranes
• Non-toxigenic strains are non-pathogenic

Mycobacterium

• Genus Mycobacterium comprises "acid-fast bacilli" difficult to stain
• Cannot be stained by simple stains due to high lipid content
• Stained by Ziehl-Neelsen stain, they appear as thin pink rods arranged singly or in small groups
• Grow on egg-enriched media such as Lowenstein-Jensen (L-J) medium
• M. tuberculosis and M. bovis: cause human tuberculosis
  • M. tuberculosis: infects via respiratory route
  • M. bovis: infects via oral route (ingestion of infected milk)
• Grow very slowly; no growth appears for 2-4 weeks at 37°C
• Tuberculin or PPD skin test (Mantoux test): delayed hypersensitivity reaction used for tuberculosis diagnosis
• M. Leprae: causes leprosy, affecting mainly the mucous membranes of the nose, skin, and nerve fibers
• Leprosy bacillus hasn't been successfully cultured on any medium
• Lepromin test

Enterobacteriaceae

• Gram-negative bacilli found in the colon of humans and other animals
• Family Enterobacteriaceae: includes many genera
• Pathogenic genera include Shigella, Salmonella, Yersinia, and some Escherichia strains.
• Opportunistic pathogens include Klebsiella, Enterobacter, Serratia, Citrobacter, Proteus, and Escherichia
• Categorized according to lactose fermentation
  • Lactose fermenters: Escherichia, Klebsiella, Citrobacter, Enterobacter
  • Lactose non-fermenters: Salmonella, Shigella, Yersinia, Proteus

Escherichia coli

• Normal inhabitant of the intestines of humans and animals
• Some E. coli strains cause urinary tract infections; produce hemolysis on blood agar
• Gram-negative bacilli, grow on simple media
• Produce rose-pink colonies on MacConkey's medium due to lactose fermentation

Diseases Caused by E. coli

• Commonest cause of urinary tract infections
• Cause wound infections, appendicitis, bacteremia, and endotoxic shock
• Enteropathogenic E. coli (EPEC) cause gastroenteritis in children
• Enterotoxigenic E. coli (ETEC), enteroinvasive E. coli (EIEC), and enterohemorrhagic E. coli (EHEC) O157:H7 cause diarrhea in all ages

Diseases Caused by Klebsiella

• K. pneumoniae: causes lobar pneumonia in humans
• Causes urinary tract infections
• K. rhinoscleromatis: causes rhinoscleroma (a granulomatous lesion in the nose and throat)
• K. oxytoca: causes hospital-acquired infections

Salmonella

• Gram-negative bacilli, non-capsulated
• S. Typhi, S. Paratyphi A, B, and C: cause enteric fever
• S. Typhimurium, S. Enteritidis, and others cause Salmonella food poisoning or enterocolitis
• S. Choleraesuis: causes septicemia with metastatic abscesses
• Widal test

Shigella

• Gram-negative bacilli, non-capsulated
• Members of the genus Shigella cause bacillary dysentery
• Manifestations include abdominal pain, fever, diarrhea with pus, blood, and mucous in stools
• Infection leads to local ulcers in the intestine

Proteus

• Gram-negative bacilli, non-capsulated
• Proteus organisms are found in the stools of humans and animals
• Cause wound infections, urinary tract infections, and septicemia (potentially life-threatening infection with large amounts of bacteria in the blood)
• Two important species: Pr. mirabilis and Pr. vulgaris

Pseudomonas

• Gram-negative bacilli, non-spore-forming
• P. aeruginosa: the commonest human pathogen in the Pseudomonas group
• Causes urinary tract infections, wound infections, corneal ulcers (in contact lens users), pneumonia, sepsis with ecthyma gangrenosum in the skin, osteomyelitis, and endocarditis (in intravenous drug users)

Vibrios, Campylobacter, and Helicobacter

• Vibrio cholerae: gram-negative bacilli causing cholera-like disease
• Campylobacter: small gram-negative rods; C. jejuni causes 95% of campylobacter enterocolitis, especially in children
• Helicobacter pylori: causes gastritis and peptic ulcers; infection is a risk factor for gastric cancer

Haemophilus

• Gram-negative small coccobacilli
• H. influenzae: found in normal throats
• Capsulated strains cause meningitis and epiglottitis in children
• Also causes complications in respiratory diseases (e.g. bronchitis, pneumonia, sinusitis) in patients with viral influenza

Clostridium

• Gram-positive bacilli
• Most species are found in the intestine of humans and animals
• C. tetani: causes tetanus in humans and animals
• C. perfringens: causes gas gangrene (gas in tissues)
• C. botulinum: causes food poisoning
• C. difficile: causes enterocolitis

This quiz explores the characteristics, infections, and medical significance of pathogenic bacteria, focusing on Staphylococci and Streptococci. Learn about the differences in their classification, the infections they cause, and their implications for health. Understand the pathogenic nature and clinical relevance of these bacteria in a medical context.