Exhaustive Study Guide: Aerobic Gram-Positive Rods

General Characteristics and Identification of Gram-Positive Rods

  • Microscopic Appearance and Staining

    • Gram-Positive Rods vs. Gram-Negatives: Unlike gram-negative bacteria, which are often randomly scattered, gram-positive rods exhibit specific organizational patterns like branching or palisading.

    • Endospore Identification: Spores do not take up standard Gram stains, appearing as empty or clear areas within the rod.

    • Malachite Green Stain: This specific stain is used to identify spores, which appear green under the microscope.

    • Counterstaining: The vegetative cells surrounding the spores typically stain pink significantly using a counterstain such as safranin or carbolfuchsin.

  • Growth Patterns

    • Branching: This pattern resembles tree branches or roots extending in multiple directions. It is a defining characteristic of certain genera like Nocardia and Streptomyces.

    • Palisading: This refers to bacteria lying side-by-side. This arrangement is also described as "coryneform" or resembling "Chinese letters."

  • Biochemical Classification: The Catalase Test

    • Definition: The catalase test determines if an organism possesses the enzyme catalase, which breaks down hydrogen peroxide (H2O2H_2O_2).

    • Chemical Reaction: H2O2→H2O+O2H_2O_2 \rightarrow H_2O + O_2.

    • Positive Result: Rapid effervescence (bubbles) indicates the release of oxygen gas (O2O_2).

    • Negative Result: No bubbles are produced.

Spore-Forming Gram-Positive Rods: Bacillus Species

  • General Bacillus Characteristics

    • Morphology: Large, "chunky" rectangular rods.

    • Physiology: Catalase positive and non-branching.

    • Environment: Commonly found in soil.

  • Bacillus anthracis (Anthrax)

    • Pathogenesis and Clinical Forms:

      • Cutaneous Anthrax: Makes up approximately 99%99\% of cases. It is typically non-lethal and characterized by a large, nasty sore with a black center of dying tissue called an Eschar. It is often associated with handling contaminated animal products like sheep wool.

      • Inhalation Anthrax (Woolsorter's Disease): Associated with breathing in spores from virgin (untreated) wool. Spores travel to lymph nodes, germinate, and produce toxins causing necrosis, respiratory distress, shock, and death. It has a high mortality rate even with antibiotics.

      • Gastrointestinal Anthrax: Rare and high mortality. Caused by ingesting spores in contaminated meat (e.g., mutton/lamb). Spores are not killed by standard cooking temperatures.

      • Injection Anthrax: Extremely rare; associated with intravenous drug use.

    • Virulence Factors (Toxins):

      • Protective Antigen (PA): Binds specifically to host cells, particularly in the lungs.

      • Edema Factor (EF): Causes swelling and fluid retention (edema) in the lungs.

      • Lethal Factor (LF): Kills cells in the area of infestation.

    • Bioterrorism Status: Classified by the CDC as an agent of bioterrorism due to airborne spores that can be distributed through ventilation systems. A notable incident occurred in 2001 when spores sent via the US postal system caused 22 cases and 5 deaths.

    • Laboratory Identification:

      • Colony Morphology: On blood agar, they appear light gray or white, non-hemolytic, and have a thick, tenacious, "egg white" or "meringue" consistency. Colonies often show projections described as a Medusa head appearance.

      • Motility: Non-motile.

      • Biochemistry: Lecithinase positive (tested on egg yolk agar).

    • Treatment and Prophylaxis:

      • Primary Treatment: Ciprofloxacin or Doxycycline.

      • Other Options: Gentamicin, Erythromycin, Tetracycline, and Chloramphenicol (noting that Chloramphenicol can cause pancytopenia).

      • Prophylaxis: Ciprofloxacin or Doxycycline is administered after suspected exposure.

  • Bacillus cereus

    • Clinical Significance: Primarily known for causing food poisoning.

      • Diarrheal Form: Associated with contaminated meat; onset takes about 12 hours and lasts for 24 hours.

      • Emetic Form (Vomiting): Associated with contaminated rice (fried or steamed). It has a very rapid onset and duration.

    • Environmental Sources: Found in rice, contaminated meat, and laboratory water baths if not cleaned regularly.

    • Laboratory Identification:

      • Morphology: Large, Beta-hemolytic colonies (complete lysis of red cells creating a clear/yellowish zone).

      • Appearance: Often described as having a "frosted" or "spray snow" look on agar.

      • Motility: Motile.

    • Treatment: Usually self-limiting; treatment involves electrolyte replacement. The organism is resistant to penicillin because it produces beta-lactamase. If treated, Vancomycin or Clindamycin is used.

Branching Non-Spore-Forming Gram-Positive Rods

  • Nocardia Species

    • Characteristics: These organisms are found in soil and environment; they primarily affect immunocompromised patients.

    • Infections:

      • Pulmonary: Inhaled from dust/soil, causing pneumonia and acute or chronic lung abscesses containing thick, yellow, purulent infection.

      • Cutaneous: Caused by traumatic inoculation (e.g., getting stuck by something). Forms a nasty skin abscess called actinomycotic mycetoma, characterized by swelling, draining, and the release of yellow sulfur granules.

    • Laboratory Identification:

      • Gram Stain: Beaded appearance that can be misconstrued as cocci (GPCGPC), but with distinct branching.

      • Acid-Fast Stain: Partially acid-fast; carbolfuchsin turns them pink.

      • Growth: Takes 2 to 6 days to grow (similar to fungus). On blood agar, they look like "bread crumbs" or cauliflower.

    • Treatment: Sulfonamides. Antifungals are ineffective because these are bacteria, not fungi.

  • Streptomyces

    • Characteristics: Branched and beaded appearance; found in soil.

    • Appearance: Colonies look significantly like fungal cultures.

    • Clinical Significance: Rarely associated with human infections.

Non-Branching, Non-Spore-Forming, Catalase-Positive Rods

  • Corynebacterium Species (Diphtheroids)

    • General Traits: Normal flora of the skin and a frequent contaminant in blood cultures. They are non-motile and exhibit palisading (coryneform) or "Chinese letter" arrangements.

    • Corynebacterium diphtheriae:

      • Pathogenesis: Causes Diphtheria. Respiratory diphtheria involves a slow-onset sore throat leading to tissue necrosis and the formation of a thick membrane across the throat that can cause suffocation. It produces a characteristic, horrific odor.

      • Virulence Factor: The tox gene produces a toxin that blocks protein synthesis, leading to cell death.

      • Laboratory Identification: Pleomorphic, club-shaped rods. When grown on cysteine tellurite blood agar, colonies turn brown or black. Usually Beta-hemolytic.

      • Treatment: Combination of antitoxin and antibiotics (Penicillin or Erythromycin). Antibiotics alone are insufficient to neutralize the toxin.

      • Prevention: Vaccine-preventable via the DTaP (children) or Tdap (adults) vaccine.

  • Listera monocytogenes

    • Epidemiology: Found everywhere; common cause of food poisoning. Primarily affects newborns, the elderly, and pregnant women (can cause fetal loss/stillbirth).

    • Transmission: Associated with raw milk, soft cheeses (e.g., Brie), poultry, and processed deli/lunch meats.

    • Clinical Disease: Neonatal meningitis and septicemia. Note: Streptococcus agalactiae (Group B Strep) is another major cause of neonatal meningitis.

    • Laboratory Identification:

      • Growth: Unique ability to grow in refrigerated temperatures.

      • Morphology: Small, translucent, Beta-hemolytic colonies.

      • Motility: Demonstrates "tumbling" motility in wet preps. In tube media at room temperature, it forms an "umbrella" or "mushroom" shape.

      • Biochemistry: CAMP positive and Bile Esculin positive (turns the media black).

    • Treatment: Ampicillin or other penicillins, or macrolides.