Staphylococcus aureusGram-positive, β-hemolytic, catalase ⊕, coagulase ⊕, cocci in clusters.- Protein A (virulence factor) binds Fc-IgG, inhibiting complement activation and phagocytosis.- Forms fibrin clot around self → ...
Staphylococcus epidermidisGram ⊕, catalase ⊕, coagulase ⊝, urease ⊕ cocci in clusters.- Novobiocin sensitive.- Does not ferment mannitol (vs S aureus).- Normal flora of skin; contaminates blood cultures. - Infects prosthetic ...
Staphylococcus saprophyticus- Gram ⊕, catalase ⊕, coagulase ⊝, urease ⊕, cocci in clusters.- Novobiocin resistant.- Normal flora of female genital tract and perineum. - Second most common cause of uncomplicated UTI in young ...
Streptococcus pneumoniae- Gram ⊕, lancet-shaped diplococci. Encapsulated. IgA protease. - Optochin sensitive.- Quellung-reaction-positive (capsule swelling on microscopy after exposure to capsular antigens) Most common cause ...
Viridans group streptococciGram ⊕, α-hemolytic cocci.- Resistant to optochin (vs S pneumoniae).- Normal flora of the oropharynx. - Streptococcus mutans and S mitis cause dental caries. - S sanguinis makes dextrans that bind ...
Streptococcus pyogenes (group A streptococci)Gram ⊕ cocci in chains. - Bacitracin sensitive, β-hemolytic, pyrrolidonyl arylamidase (PYR) ⊕.- Hyaluronic acid capsule and M protein inhibit phagocytosis. Cause:- Pyogenic – pharyngitis, cellulitis, ...
Streptococcus agalactiae (group B streptococci)Gram ⊕ cocci, β-hemolytic, bacitracin resistant, colonizes vagina. - Causes pneumonia, meningitis, and sepsis, mainly in babies - Produces CAMP factor, which enlarges the area of hemolysis formed ...
Streptococcus bovisGram ⊕ cocci, γ-hemolysis, grows in bile, but does no growth in 6.5% NaCl. - Colonizes the gut. - S gallolyticus (S bovis type 1) can cause bacteremia and subacute endocarditis and is associated with ...
EnterococciGram ⊕ cocci. Catalase ⊝, PYR ⊕ (like group A streptococcus)- γ-hemolysis. Can grow in 6.5% NaCl and bile (vs Streptococcus bovis).- Enterococci (E faecalis and E faecium) are normal colonic ...
Bacillus anthracisGram ⊕, spore-forming rod that produces anthrax toxin.- Only bacterium with an antiphagocytic polypeptide capsule (contains D-glutamate).- Colonies show halo of projections, sometimes referred to as ...
Bacillus cereus- Gram ⊕ rod. Spores are heat-stable.- Grows in heated food that cools down too slowly or is improperly refrigerated (also known as reheated rice syndrome).- Keeping rice warm results in germination ...
Corynebacterium diphtheriaeGram-⊕, aerobic rod.- Transmitted by respiratory droplets. - Causes diphtheria via exotoxin encoded by β-prophage. Potent exotoxin inhibits protein synthesis via ADP-ribosylation of EF-2. Symptoms:- ...
Listeria monocytogenesGram ⊕, catalase ⊕, motile, facultative intracellular rod.- Acquired by ingestion of unpasteurized dairy products and cold deli meats, via transplacental transmission, or by vaginal transmission during ...
Leprosy (Hansen disease)- Caused by Mycobacterium leprae, an obligate intracellular, acid-fast bacillus - Likes cold temperatures (infects skin and superficial nerves – "glove and stocking" sensation).- Cannot be cultured.- ...
NeisseriaGram ⊝ diplococci. Metabolize glucose and produce IgA proteases. Contain lipooligosaccharides (LOS) with strong endotoxin activity. N gonorrhoeae is often intracellular (within neutrophils). Meningococci- ...
Haemophilus influenzaeSmall gram ⊝ coccobacillary rod. - Aerosol transmission. - Produces IgA protease. - Culture on chocolate agar, which contains factors V (NAD+) and X (hematin) for growth. - Can also be grown with S ...
Bordetella pertussisGram ⊝, obligate aerobic coccobacillus.- Attachment to nasopharyngeal ciliated epithelial cells is via hemagglutinin; pertussis toxin aids in attachment.- Pertussis toxin (A and B component): ADP ribosylation ...
Legionella pneumophilaGram ⊝ rod, facultative intracellular, obligate aerobe.- Aerosol transmission from environmental water source habitat (eg, air conditioning systems, hot water tanks). No person-to-person transmission.- ...
Pseudomonas aeruginosaGram ⊝, aerobic, motile. Non-lactose fermenter. Oxidase ⊕.- Grows in water and humid conditions, eg, hot tub, contaminated lens solution.- Has grape-like odor. - Produces pyoverdin and pyocyanin ...
E coliGram-negative rod. Produces β-galactosidase, which breaks down lactose into glucose and galactose. EMB agar – lactose fermenters grow as purple/black colonies. E coli grows colonies with a green sheen. ...
KlebsiellaGram ⊝ rod. - Polysaccharide capsule.- Reservoir: human colon and upper respiratory tract. - Mucoid, lactose-fermenting colonies on MacConkey agar.- Dark red "current jelly" sputum (blood/mucus) - ...
Campylobacter jejuniGram ⊝, oxidase ⊕. Comma-shaped rod with polar flagella. Grows at 42°C.- Reservoir: Intestinal tracts of humans, cattle, sheep, dogs, cats, poultry- Transmission: Fecal-oral transmission through ...
Clostridium tetaniGram-⊕, spore-forming, obligate anaerobic rods.- Produces tetanospasmin- Ubiquitous (especially animal feces and soil) - Wounds with compromising blood supply create anaerobic conditions that are ideal ...
Clostridium botulinumGram-⊕, spore-forming, obligate anaerobic rod. - Produces a heat-labile toxin that inhibits ACh release at the neuromuscular junction, causing botulism. - In adults, disease is caused by ingestion of ...
Clostridium perfringensGram-⊕, spore-forming, obligate anaerobic rod.- Ubiquitous - Wounds with compromised blood supply create an anaerobic environment → optimal for the proliferation of C perfringens → necrosis progresses ...
Clostridium difficileGram-⊕, spore-forming, obligate anaerobic rod. Produces 2 toxins:- Toxin A, an enterotoxin, binds to the brush border of the gut and alters fluid secretion.- Toxin B, a cytotoxin, causes cytoskeletal ...
Vibrio choleraeGram ⊝, flagellated, oxidase ⊕, grows in alkaline media.- Endemic to developing countries.- Transmitted via ingestion of contaminated water or uncooked food (eg, raw shellfish). - Fecal-oral spread; ...
Helicobacter pyloriGram ⊝, curved, flagellated (motile), triple ⊕: catalase ⊕, oxidase ⊕, urease ⊕- Urease produces ammonia, creating an alkaline environment, which helps H pylori survive in acidic mucosa.- ...
SpirochetesSpiral-shaped bacteria with axial filaments. - Borrelia: only spirochete visible in light microscopy due to big size; aniline dyes (Wright or Giemsa stain) - Leptospira - Treponema: dark-field microscopy ...
Leptospira interrogansSpirochete with hook-shaped ends.- Zoonosis. - Found in water contaminated with animal urine.- Prevalent among surfers and in tropics (eg, Hawaii). Leptospirosis- Flu-like symptoms - Myalgias (classically ...
Lyme disease- Caused by Borellia burgdorferi, an anaerobe facultative intracellular spirochete, which is transmitted by the Ixodes deer tick- Natural reservoir is the mouse- Common in northeastern United States ...
Syphilis- Caused by spirochete Treponema pallidum Primary syphilis: Painless chancre. VDRL ⊕ in 80%, dark-field microscopy. Secondary syphilis: Maculopapular rash (including palms and soles), condylomata ...
Congenital syphilis- facial abnormalities such as rhagades (linear scars at angle of mouth), snuffles (nasal discharge), notched (Hutchinson) teeth, deafness - To prevent, treat mother early in pregnancy, as placental transmission ...
VDRL false positives- VDRL detects nonspecific antibody that reacts with beef cardiolipin. - Widely available test for syphilis, quantitative, sensitive but not specific. False-positive results on VDRL with:V: Viral infection ...
Jarisch-Herxheimer reactionFlu-like syndrome (fever, chills, headache, myalgia) after antibiotics are started. Due to killed bacteria (usually spirochetes: Borrelia, Leptospira, Treponema) releasing toxins.
Gardnerella vaginalisPleomorphic, gram-variable rods. Catalase and oxidase negative. - Involved in bacterial vaginosis (excessive growth of certain anaerobic bacteria in vagina). - Thin, gray vaginal discharge with a fishy ...
Q feverZoonosis caused by Coxiella burnetti, no arthropod vector - Spores inhaled as aerosols from cattle/sheep amniotic fluid, animal waste Acute Q fever:- Nonfebrile illness with fever lasting >10 days- Fatigue, ...
Rickettsial diseasesRocky Mountain spotted fever- Rickettsia rickettsii, vector is Dermacentor (dog tick)- Occurs primarily on the East Coast (esp. North Carolina)- Rash typically starts at wrists and ankles and then spreads ...
ChlamydiaeObligate intracellular organisms (cannot make their own ATP) with lack of peptidoglycan (muramic acid) in cell wall. Chlamydia trachomatisTypes A, B, C: chronic infection, blindness due to follicular ...
Mycoplasma pneumoniaePleomorphic, microaerophile, no cell wall (contains sterols for stability). Not seen on Gram stain.- Frequent outbreaks in military recruits and prisons, <30 years old. - Classic cause of "walking" atypical ...
Systemic mycosesAll can cause pneumonia and can disseminate.All are caused by dimorphic fungi: cold (20°C) = mold; heat (37°C) = yeast. Only exception is Coccidioides, which is a spherule (not yeast in tissue).Systemic ...
Candida albicansDimorphic; forms pseudohyphae and budding yeasts at 20°C, germ tubes (true hyphae) at 37°C serum. Forms pseudohyphae when it invades tissues. This test helps differentiate C albicans from other Candida ...
Aspergillus fumigatusMonomorphic septate hyphae that branch at 45°. Produces conidia in radiating chains at the end of conidiophore (like broccoli). - Causes invasive aspergillosis in pre-existing lung cavities, especially ...
Cryptococcus neoformans- Opportunistic infection in patients with CD4 <100/mm3.- 5-10 µm round/oval cells with narrow-based budding. Heavily encapsulated yeast with a polysaccharide capsule. Not dimorphic (grows as a yeast ...
Pneumocystis jiroveciiCauses Pneumocystis pneumonia (PCP), a diffuse interstitial pneumonia.- Yeast-like fungus.- Most infections are asymptomatic.- Immunosuppression (eg, AIDS) predisposes to disease. - Diffuse, bilateral ...
Sporothrix schenckiiDimorphic, cigar-shaped budding yeast that grows in branching hyphae with rosettes of conidia; lives on vegetation.- Spores are traumatically introduced into the skin, typically by a thorn ("rose gardener's disease") ...
Toxoplasma gondiiProtazoa. - Immunocompetent: mononucleosis-like symptoms, ⊝ heterophile antibody test.- Reactivation in AIDS → brain abscesses usually seen as multiple ring-enhancing lesions on MRI.- Congenital ...
Trypanosoma cruziChagas disease – dilated cardiomyopathy with apical atrophy, megacolon, megaesophagus, achalasia.- Acute stage: Romaña sign (unilateral periorbital swelling). - Predominantly in South America- ...
Giardia lambliaProtazoa.Intestinal flagellate that exists in 2 forms: a trophozoite (pathogenic stage) or a cyst (infective stage). - The trophozoite is a bilaterally symmetric, pear-shaped oragnism with multiple flagella ...