Microbiology Test 4

Staphylococcus aureus

• Gram-positive

• Cocci

• Coagulase positive

• Causes pyogenic (pus-forming) and toxin-mediated infections

• Produces TSST-1 → toxic shock syndrome

• Produces exfoliative toxin → impetigo and scalded skin syndrome

• Associated with boils, abscesses, wound infections, and food poisoning

Staphylococcus epidermidis

• Gram positive

• Cocci

• Coagulase-negative

• Iatrogenic infections (catheters, prosthetics, incision sites)

• forms biofilms on medical devices

• Normal flora of the skin

• Opportunistic pathogen, especially in hospitalized patients

Staphylococcus saprophyticus

• Gram positive

• Cocci

• Coagulase-negative

• Causes UTIs, especially in sexually active young women

• Resistant to novobiocin (key diagnostic test)

• Normal flora of genitourinary tract

• Non-hemolytic on blood agar

Staphylococcus lugdunensis

• Gram positive

• Cocci

• Coagulase-negative

• Can cause acute endocarditis

• More virulent than most CoNS (coagulase-negative staph)

• β-hemolytic

• Often associated with skin and soft tissue infections

Streptococcus pyogenes (Group A Strep/ GAS)

• Gram positive

• Cocci

• β-hemolytic on blood agar

• M protein is the main virulence factor (anti-phagocytic, type-specific immunity)

• Causes pharyngitis, impetigo, erysipelas, scarlet fever, necrotizing fasciitis

• Can lead to autoimmune sequelae → rheumatic fever, glomerulonephritis

• Produces streptolysins, hyaluronidase, and erythrogenic toxin

Streptococcus agalactiae (Group B Strep/ GBS)

• Gram positive

• Cocci

• β-hemolytic

• Commonly found in neonatal sepsis and meningitis

• Colonizes female genital tract

• CAMP test positive

• Resistant to bacitracin (unlike GAS)

Streptococcus pneumoniae

• Gram positive

• Cocci

• α-hemolytic

• Capsulated diplococcus (lancet-shaped)

• Optochin sensitive and bile soluble

• Causes pneumonia, meningitis, otitis media, sinusitis

• Major virulence factor = polysaccharide capsule

Staphylococcus intermedius

• Gram positive

• Cocci

• Coagulase positive

• Zoonotic pathogen (commonly found in dogs)

• Associated with dog bite wounds in humans

• Can cause skin and soft tissue infections, wound abscesses, and occasionally bacteremia

• beta-galactosidase positive

• mannitol negative

Viridans Streptococci

• Gram positive

• Cocci

• alpha-hemolytic

• Optochin resistant

• Normal flora of the oral cavity

• Cause dental caries and subacute endocarditis

• Lack Lancefield antigens

Enterococcus faecalis / Enterococcus faecium

• Gram positive

• Cocci

• Group D streptococci

• Grow in 6.5% NaCl and bile esculin positive

• Cause UTIs, endocarditis, wound infections

• Normal flora of the gut

• Antibiotic resistant

Bacillus anthracis

• Gram positive

• Rod shaped

• Aerobic

• Endospore-forming

• produces anthrax toxin

• forms black eschar 

• Non motile

• Encapsulated with poly-D-glutamate capsule

Bacillus cereus

• Gram positive

• Rod

• Aerobic

• Endospore forming

• Causes food poisoning

• heat stable enterotoxin

• Motile

• beta-homolytic

Clostridium botulinum

• Gram positive

• Rod

• Anaerobic

• Endospore forming

• Produces botulinum toxin (BoNT) → flaccid paralysis

• Blocks ACh release at neuromuscular junction

• Commonly from canned foods or honey(infant botulism)

• Neurotoxin is heat labile

Clostridium tetani

• Gram positive

• Rod

• Anaerobic

• Endospore forming

• Produces tetanus toxin (TeNT) → spastic paralysis

• Blocks release of inhibitory neurotransmitters (GABA, glycine)

• Causes “lockjaw” (aka trismus) and muscle spasms

• Common in puncture wounds contaminated with soil

Clostridium perfringens

• Gram positive

• Rod

• Anaerobic

• Endospore forming

• Produces alpha-toxin (lecithinase) that destroys cell membranes

• Double zone of hemolysis on blood agar

• Found in contaminated wounds and food

Clostridium difficile

• Gram positive

• Rod

• Anaerobic

• Endospore forming

• Causes antibiotic-associated diarrhea and pseudomembranous colitis

• Toxin A (enterotoxin) and Toxin B (cytotoxin)

• Often follows broad-spectrum antibiotic use

• Diagnosed by toxin detection in stool

Listeria monocytogenes

• Gram positive

• Rod

• Non-spore forming

• Causes listeriosis (meningitis in neonates, immunocompromised)

• Motile (tumbling motility)

• Intracellular pathogen → uses Listeriolysin O (LLO) to escape phagosomes

• Transmitted via unpasteurized dairy or cold meats

Corynebacterium diphtheriae

• Gram positive

• Rod

• Aerobic

• Non spore forming

• Causes diptheria via diptheria toxin (inhibits EF-2)

• Forms pseudomembrane in throat

• Nonmotile

• Catalase positive

• Humans are the only reservoir

Propionibacterium acnes

• Gram positive

• Rod

• Anaerobic

• Non spore forming

• Normal skin flora

• Associated with acne vulgaris

• Can infect prosthetic devices and catheters

• Produces propionic acid from sebum

Actinomyces spp.

• Gram positive

• Rod

• Anaerobic

• Filamentous

• Normal oral flora

• Causes actinomycosis (chronic abscesses with sulfur granules)

• Opportunistic after dental procedures or trauma

• Non-acid fast

Mycobacterium tuberculosis

• Gram positive

• Acid fast

• Mycolic acid cell wall

• Causes tuberculosis (TB)

• Transmission by aerosols

• Infects lungs (pulmonary TB)

• Can remain latent (dormant) for years

• Forms caseating granulomas

• Diagnosed via acid-fast stain or tuberculin test

Mycobacterium leprae

• Gram positive

• Rod

• Acid fast

• Bacillus

• Causes leprosy (Hansen’s disease)

• Obligate intracellular pathogen (cannot grow in vitro)

• Affects skin, peripheral nerves, and eyes

• Transmitted via prolonged close contact

Nocardia spp.

• Gram positive

• Partially acid fast

• Filamentous

• Causes nocardiosis (lung and brain abscesses, cutaneous infections

• Found in soil

• Opportunistic in immunocompomised patients

Neisseria meningitidis

• Gram negative

• Cocci

• Aerobic

• Nonmotile

• Diplococcus (“kidney bean”) shape

• Oxidase-positive, distinguishing feature of Neisseria spp.

• Causes meningitis and meningococcal septicemia

• Possesses a polysaccharide capsule that resists phagocytosis

• Transmitted via respiratory droplets

Neisseria gonorrhoeae

• Gram negative

• Cocci

• Aerobic

• Oxidase-positive

• Diplococcus

• Causes gonorrhea, ophthalmia neonatorum, and PID

• No capsule; survives by antigenic variation of pili and Opa proteins

• Intracellular (found inside neutrophils on Gram stain)

• Transmitted sexually or during birth

Escherichia coli

• Gram negative

• Rods

• Facultative anaerobe

• Lactose-fermenting coliform; oxidase negative

• Causes gastroenteritis, UTIs, and neonatal meningitis

• EHEC (O157:H7) strain produces Shiga-like toxin → HUS

• Possesses type III secretion system aiding invasion

• Found in contaminated beef, milk, or juices 

• Enterobacteriaceae

Klebsiella pneumoniae

• Gram negative

• Rod

• Facultative anaerobe

• Lactose fermenting

• Coliform

• Large polysaccharide capsule

• Causes pneumonia in elderly or immunocompromised patients

• Can also cause bacteremia, UTIs, meningitis, wound infections

• “String test positive” due to mucoid capsule

• Carbapenem-resistant strains (CRE) are 

• Enterobacteriaceae

Serratia marcescens

• Gram negative

• Rod

• Facultative anaerobe

• Produces red pigment when grown at room temperature.

• Found in catheters, saline, hospital supplies.

• Causes opportunistic UTIs and respiratory infections.

• Often antibiotic resistant.

• Common in nosocomial (hospital-acquired) settings.

• Enterobacteriaceae

Enterobacter, Hafnia, Citrobacter

• Gram negative

• Rod

• Facultative anaerobe

• Lactose-fermenting coliforms found in soil, water, and intestines.

• Opportunistic in immunocompromised and hospital patients.

• Cause respiratory, urinary, or wound infections.

• Often multidrug resistant.

• Normal flora turned pathogenic by medical procedures.

• Enterobacteriaceae

Proteus mirabilis

• Gram negative

• Rod

• Facultative anaerobe

• Non-lactose fermenter, motile swarmer on agar.

• Causes UTIs (especially with long-term catheters).

• Produces urease, leading to kidney stones (alkaline urine).

• Has a strong “fishy” odor on plates.

• Resistant to many antibiotics.

• Enterobacteriaceae

Morganella, Providencia, Edwardsiella

• Gram negative

• Rod

• Facultative anaerobe

• Non-lactose fermenting Enterobacteriaceae.

• Cause nosocomial UTIs in immunocompromised hosts.

• Opportunistic infections similar to Proteus spp.

• Some species resistant to multiple drug classes.

• Enterobacteriaceae

Salmonella enterica

• Gram negative

• Rod

• Motile

• H2S positive

• Non lactose fermenter

• S. typhi causes typhoid fever (high fever, rash, GI ulcers).

• S. typhimurium causes salmonellosis (food poisoning).

• Infection via contaminated poultry, eggs, or water.

• Type III secretion system aids epithelial invasion.

Shigella spp.

• Gram negative

• Rod

• Non motile

• non lactose fermenting

• H2S negative

• Cause shigellosis (bacillary dysentery) → bloody/mucous diarrhea.

• S. dysenteriae produces Shiga toxin (inhibits protein synthesis).

• Humans are the only host.

• Spread via fecal–oral route, often in crowded settings.

Yersinia pestis

• Gram negative

• Rod

• Non-motile

• Bipolar-staining “safety pin” appearance

• Causes plague (bubonic & pneumonic) — Category A bioterrorism agent.

• Reservoir: rodents & fleas; can spread aerosol (pneumonic form).

• Untreated mortality up to 100% (pneumonic).

• Treated with streptomycin or gentamicin rapidly.

Yersinia enterocolitica

• Gram negative

• Rod

• Motile at 25°C, non-lactose fermenting.

• Transmitted by contaminated food or water.

• Causes enterocolitis resembling appendicitis.

• Zoonotic (pigs and rodents reservoirs).

• Can persist in cold food (refrigerated meats).

Haemophilus influenzae

• Gram negative

• Coccobacilli 

• Aerobic

• Nonfermenters

• Small pleomorphic coccobacillus, requires X (heme) and V (NAD⁺) factors.

• Grows on chocolate agar, forms satellite colonies around Staph.

• Encapsulated type b (Hib) causes meningitis, epiglottitis.

• Other strains cause otitis media, sinusitis, bronchitis.

• Prevented by Hib vaccine.

Bartonella henselae / B. quintana

• Gram negative

• Rod

• Facultative

• Intracellular

• B. henselae → Cat scratch fever (cats are reservoir).

• B. quintana → Trench fever (spread by body lice).

• Infect immunocompromised patients (e.g., HIV).

• Persistent bloodstream infection possible.

Brucella abortus

• Gram negative

• Aerobic

• Coccobacillus

• Nonmotile

• Small

• Causes brucellosis (“undulant fever”).

• Acquired from unpasteurized dairy or infected animals.

• Zoonotic and can cause miscarriage in animals.

• Intracellular pathogen, difficult to eradicate.

Bordetella pertussis

• Gram negative

• Aerobic 

• Coccobacillus

• Small

• Nonmotile

• Causes whooping cough (pertussis).

• Produces pertussis toxin and adhesins damaging respiratory epithelium.

• Spread via aerosols; mainly affects children.

• Prevented by DTaP vaccine.

Borrelia burgdorferi

• Gram negative

• Spirochete

• Detected by: Dark-field microscopy (limited use), Two-step serology: ELISA → Western blot confirmation, PCR from joint fluid or CSF (Lyme disease cases)

• Culture rarely used (slow and difficult)

• Transmitted by Ixodes ticks

Treponema pallidum

• Thin, motile

• Spirochete

• Detected by: Dark-field microscopy (primary lesions)

  Nontreponemal tests: VDRL, RPR

  Treponemal tests: FTA-ABS, TP-PA

• Cannot be cultured in vitro

• Motility observed with dark0field or DFA

Campylobacter jejuni

• Gram negative

• Curved shaped rod

• , grows best at 42°C (“likes the campfire”)

• Oxidase positive, catalase positive

• Motile (darting motility)

• Hippurate hydrolysis positive

• Commonly isolated from selective media: Skirrow or Campy agar

Helicobacter pylori

• Gram negative

• Spiral shaped rod

• Polar flagella

• Urease positive (Key test — rapid urease test on biopsy)

• Oxidase positive, catalase positive

• Detected by: Urea breath test, stool antigen test, histology of gastric biopsy (Warthin-Starry stain)

• Microaerophilic grows on Skirrow or Campy media

Vibrio cholerae

• Comma-shaped

• Motile

• Oxidase-positive rod

• Grows in alkaline media

• Ferments sucrose

• String test positive

• Detected in stool by culture → yellow colonies

• Agglutination with o1/o139 antisera

Vibrio parahaemolyticus

• Halophilic (requires salt)

• Oxidase positive, motile

• TCBS agar: green colonies (no sucrose fermentation)

• Causes gastroenteritis from raw seafood

• Resistant to 0% NaCl growth (needs 1–3%)

Vibrio vulnificus

• Halophilic, oxidase positive, ferments lactose

• Grows on TCBS agar (green colonies)

• Sensitive to bile salts

• Detected in wound or blood cultures

• Causes wound infections, necrotizing fasciitis, and septicemia (esp. with shellfish exposure)

Chlamydia trachomatis

• Obligate intracellular, lacks peptidoglycan

• Cannot grow on artificial media

• Detected by: NAAT (PCR) on urine or swab, Direct fluorescent antibody stain (elementary bodies), Inclusion bodies on Giemsa stain

• Nonmotile, Gram indeterminate

• Culture on McCoy cells or HeLa cells possible (rare)

Chlamydia psittaci

• Obligate intracellular, similar to C. trachomatis

• Reservoir: birds (zoonotic – “parrot fever”)

• Detected by:, Serologic testing (complement fixation or micro-IF), PCR

• Cannot be cultured on routine media

• No peptidoglycan layer, Gram stain poor

Coxiella burnetii

• Very small, obligate intracellular coccobacillus

• Detected by: Serology — Phase I and II antibodies (Q fever diagnosis) & PCR

• Grows in eukaryotic cells only (not in regular culture)

• Resistant to heat and desiccation (spore-like variant)

• No arthropod vector needed (airborne transmission)

Ehrlichia spp.

• Obligate intracellular, infects leukocytes

• Detected by: Morulae (mulberry-like inclusions) in monocytes or granulocytes & PCR (most specific) , Serology (IFA)

• Cannot be cultured on standard media

• Transmitted by ticks (Amblyomma spp.)

Rickettsia spp.

• Obligate intracellular rods

• Weakly Gram-negative, stain poorly

• Detected by: Weil–Felix test (cross-reactivity with Proteus OX antigens, historical) & PCR or immunofluorescence assay (IFA) , Endothelial cell infection seen in biopsy (vasculitis)

• Cultured only in cell lines or yolk sacs

• Transmitted by arthropods (ticks, lice, fleas)

Francisella tularensis

• Small, pleomorphic coccobacillus

• Requires cysteine for growth (chocolate or buffered charcoal yeast extract agar)

• Weakly Gram-negative, oxidase negative

• Detected by: Serology (antibody rise in paired sera) & PCR

• Highly infectious (BSL-3); Category A bioterrorism agent

Haemophilus ducreyi

• Gram-negative coccobacillus, oxidase positive

• Requires X factor (heme) only; no NAD⁺ (V factor)

• Grows on chocolate agar (not on blood)

• School of fish” or “railroad track” arrangement on Gram stain

• Causes chancroid (painful genital ulcers)

Escherichia coli

• Gram-negative rod, lactose fermenter (pink on MacConkey)

• Indole positive, oxidase negative

• Methyl red positive, Voges-Proskauer negative

• EHEC (O157:H7): sorbitol non-fermenting on SMAC agar

• Serotyping via O, H, and K antigens

Salmonella spp.

• Motile, H₂S positive, non-lactose fermenting

• Black colonies on Hektoen or SS agar

• Urease negative, lysine decarboxylase positive

• Produces type III secretion system

• S. Typhi produces Vi capsular antigen

Mycoplasma pneumoniae

• No cell wall (not visible on Gram stain)

• Fried-egg colonies on Eaton’s agar (requires cholesterol)

• Cold agglutinin test positive (IgM agglutinates RBCs)

• PCR or serology for diagnosis

• Resistant to β-lactams (no peptidoglycan)

Mycoplasma genitalium

• Wall-less, pleomorphic, very small

• Difficult to culture — requires cholesterol-enriched media

• Detected by NAAT (PCR)

• Urease negative (differs from Ureaplasma)

• Causes NGU (nongonococcal urethritis) and PID

Ureaplasma urealyticum

• No cell wall, pleomorphic

• Urease positive (key distinguishing test)

• Grows on A8 or Shepard’s agar (requires urea and cholesterol)

• Detected by PCR or culture from urogenital specimens

• Associated with NGU and preterm birth