Non-spore formers

Non Spore Forming Gram Positive Bacilli Primary Differential Keys

Differentiated primarily based on catalase activity (catalase positive vs catalase negative) and morphological branching (branching filaments vs non branching rods).

Major Non Spore Forming Gram Positive Genera

Corynebacterium, Listeria, Erysipelothrix, Gardnerella, Lactobacillus, and aerobic actinomycetes such as Nocardia and Rhodococcus.

Corynebacterium Microscopic Morphology

Irregular, nonparallel sides with a palisading arrangement forming a characteristic Chinese letters or picket fence appearance. Highly pleomorphic with club shaped swellings.

Palisading Appearance in Corynebacterium

Arrangement resembling side by side picket fence poles or Chinese character letters caused by snapping division during binary fission.

Corynebacterium Normal Habitat

Commensal flora colonizing human skin and mucous membranes.

Diphtheroids Definition

All Corynebacterium species other than Corynebacterium diphtheriae that usually act as commensals or opportunistic pathogens.

Most Pathogenic Species of Corynebacterium

Corynebacterium diphtheriae, the primary human pathogen in the genus.

Corynebacterium Respiration and Catalase

Facultative anaerobe and strongly catalase positive.

Corynebacterium Growth on MacConkey Agar

Negative. Cannot grow on MacConkey agar because crystal violet and bile salts inhibit gram positive bacilli.

Corynebacterium diphtheriae Historical Name

Klebs Loeffler bacillus, named after the bacteriologists who discovered and isolated it.

Corynebacterium diphtheriae Hemolysis and DNase

Produces a narrow zone of beta hemolysis on sheep blood agar and tests positive for deoxyribonuclease (DNase positive).

Diphtheria Definition

An acute, contagious public health disease caused by toxigenic strains of Corynebacterium diphtheriae affecting respiratory mucous membranes or skin.

Diphtheria Clinical Symptoms

Low grade fever, general malaise, sore throat, cervical lymphadenopathy producing a bull neck appearance, and pseudomembrane formation on tonsils.

Diphtheria Pseudomembrane

A tough, adherent grayish white membrane formed on tonsils and pharynx composed of necrotic epithelial cells, fibrin, leukocytes, and bacterial colonies. Forcible removal causes bleeding.

Diphtheria Bull Neck Appearance

Marked cervical lymphadenitis and extensive soft tissue edema of the neck associated with severe respiratory diphtheria.

Diphtheria Toxin Mode of Action

A lethal heat labile AB exotoxin produced by lysogenic strains carrying the beta bacteriophage. Fragment A blocks protein synthesis by ADP ribosylating Elongation Factor 2 (EF 2), causing mucosal necrosis and tissue inflammation.

Diphtheria Primary Site of Infection

Epithelial cells of the tonsils, pharynx, larynx, and nasal passages.

DPT Vaccine

A trivalent combination vaccine containing diphtheria toxoid, pertussis antigens, and tetanus toxoid administered during infancy before one year of age.

Cystine Tellurite Blood Agar (CTBA)

Selective differential medium for Corynebacterium diphtheriae. Base medium is blood agar, and the inhibitory selective agent is potassium tellurite.

Potassium Tellurite Mechanism and Colony Appearance

Inhibits most normal upper respiratory flora. Corynebacterium diphtheriae reduces tellurite intracellularly to tellurium, forming distinctive gray black colonies.

Tinsdale Medium

Highly selective serum tellurite cystine medium for Corynebacterium diphtheriae. Organisms produce black colonies surrounded by a dark brown halo due to cysteinase activity.

Loeffler Serum Medium

Coagulated serum medium used to enhance growth, stimulate pleomorphic club morphology, and promote the formation of metachromatic granules.

Pai Coagulated Egg Medium

Coagulated whole egg medium utilized like Loeffler serum to stimulate marked granule production and distinct pleomorphism.

Babes Ernst Granules

Metachromatic intracellular storage granules composed of polymerized inorganic polyphosphates that stain reddish purple with methylene blue or Albert stain.

Elek Test (Ouchterlony Immunodiffusion)

In vitro toxigenicity test where antitoxin impregnated filter paper is placed on agar perpendicular to bacterial streak lines. Toxin and antitoxin diffuse, forming visible precipitin lines at a 45 degree angle if toxigenic.

Elek Test Positive Result

Formation of visible precipitin lines or arcs of precipitation between the bacterial growth and the antitoxin filter paper strip.

Elek Test Negative Result

Absence of precipitin arcs or precipitation lines, confirming the isolate is non toxigenic.

Animal Inoculation Toxigenicity Test

In vivo virulence assay where guinea pigs are injected intraperitoneally with bacterial suspension or toxin. Death of unprotected animal within two to three days confirms lethal toxin production.

Schick Test

In vivo skin test used historically to determine human susceptibility or immunity to diphtheria toxin. Diphtheria toxin is injected into one forearm and heated toxoid control into the other.

Schick Test Positive Interpretation

Erythema and local inflammation at the injection site peaking around day four to seven, indicating absence of circulating antitoxin and susceptibility to diphtheria.

Schick Test Negative Interpretation

No inflammatory reaction at either arm, indicating the presence of circulating neutralizing antibodies and immunity to diphtheria.

Vero Cell Cytotoxicity Assay

Tissue culture cytotoxicity test considered the gold standard reference method for detecting functional diphtheria toxin production.

Molecular and Rapid Tests for Diphtheria

Polymerase chain reaction (PCR) targeting the tox gene, enzyme linked immunosorbent assay (ELISA), and immunochromatographic strip assays.

Corynebacterium diphtheriae First Line Treatment

Immediate administration of intravenous diphtheria antitoxin to neutralize circulating toxin, combined with penicillin as the primary antibacterial drug of choice.

Corynebacterium diphtheriae Alternative Antibiotic

Erythromycin, recommended for penicillin allergic patients to eliminate bacterial colonization and prevent disease transmission.

Corynebacterium diphtheriae Biovars Definition

Four recognized biological variants possessing identical microscopic morphology but exhibiting distinct colonial and biochemical phenotypes.

Corynebacterium diphtheriae Biovar Gravis

Biotype that is urease negative, nitrate reduction positive, glycogen fermentation positive, and non lipophilic. Produces large, flat, dull gray colonies with radial striations resembling a daisy head.

Corynebacterium diphtheriae Biovar Mitis

Biotype that is urease negative, nitrate reduction positive, glycogen fermentation negative, and non lipophilic. Produces small, smooth, convex, shiny black colonies.

Corynebacterium diphtheriae Biovar Belfanti

Biotype that is urease negative, nitrate reduction negative, glycogen fermentation negative, and non lipophilic.

Corynebacterium diphtheriae Biovar Intermedius

Biotype that is urease negative, nitrate reduction positive, glycogen fermentation negative, and lipophilic. Produces tiny, flat, pinpoint black colonies.

Corynebacterium jeikeium (JK Bacillus)

Strictly aerobic, gamma hemolytic, lipophilic diphtheroid. Highly multidrug resistant hospital acquired pathogen causing septicemia, endocarditis, and prosthetic device infections.

Corynebacterium jeikeium Colonial Feature

Produces a characteristic metallic sheen on sheep blood agar and requires lipid supplementation for optimal growth.

Corynebacterium ulcerans

Animal pathogen causing bovine mastitis in cattle. Transmitted to humans through raw unpasteurized milk or contact with cattle, producing diphtheria like pharyngeal infections and ulcers.

Corynebacterium urealyticum

Slow growing, lipophilic, multidrug resistant urinary tract pathogen that is strongly urease positive within minutes. Associated with alkaline encrusted cystitis and struvite stones.

Encrusted Cystitis and Pyelitis

Severe chronic urinary tract infection characterized by mucosal inflammation and deposition of magnesium ammonium phosphate crystals driven by Corynebacterium urealyticum urease.

Corynebacterium amycolatum

The most common diphtheroid recovered from human clinical specimens. Non lipophilic opportunistic pathogen frequently implicated in prosthetic joint and foreign body infections.

Corynebacterium pseudodiphtheriticum (Hoffman Bacillus)

Normal commensal of human nasopharynx. Non lipophilic, urease positive, nitrate positive diphtheroid that can cause endocarditis on native or prosthetic heart valves.

Corynebacterium minutissimum

Causative agent of erythrasma, a superficial chronic skin infection affecting intertriginous skin folds such as the axillae, groin, and toe webs.

Erythrasma Clinical Feature and Wood Lamp

Presents as well demarcated reddish brown macular patches in moist body folds that fluoresce brilliant coral red under ultraviolet Wood lamp illumination.

Cause of Coral Red Fluorescence in Erythrasma

Production and accumulation of water soluble coproporphyrin III by Corynebacterium minutissimum colonies.

Differential Diagnosis: Erythrasma vs Erysipeloid vs Erysipelas

Erythrasma is a superficial skin fold infection caused by Corynebacterium minutissimum; Erysipeloid is an occupational zoonotic cutaneous infection caused by Erysipelothrix rhusiopathiae; Erysipelas is an acute painful dermis infection caused by Streptococcus pyogenes.

Corynebacterium pseudotuberculosis

Veterinary pathogen causing caseous lymphadenitis in sheep and goats. Urease positive, produces beta hemolysis, and expresses both diphtheria toxin and phospholipase D.

Listeria monocytogenes Taxonomy and General Traits

Small, facultatively anaerobic, non spore forming, catalase positive, gram positive coccobacillus that shows narrow zone beta hemolysis.

Listeriosis Definition

Foodborne infectious disease caused by ingestion of Listeria monocytogenes from unpasteurized dairy, soft cheeses, cold deli meats, and raw produce.

Listeriosis in Pregnant Women

Presents as a nonspecific flu like febrile illness that can trigger amnionitis, premature labor, spontaneous abortion, and intrauterine fetal death.

Listeriosis in Neonates

Causes early onset sepsis with granulomatosis infantiseptica or late onset neonatal purulent meningitis acquired during passage through the birth canal.

Listeriosis in Immunosuppressed Adults

Severe invasive opportunistic disease presenting as acute bacterial meningitis, cerebritis, bacteremia, and infective endocarditis.

Listeriolysin O (LLO)

Major virulence factor of Listeria monocytogenes. A cholesterol dependent pore forming cytolysin that dissolves the phagosomal membrane, allowing bacterial escape into host cytoplasm.

Internalin and Protein p60 in Listeria

Surface proteins that promote host cell adherence and induce phagocytic uptake into non phagocytic mammalian cells.

ActA Protein in Listeria

Bacterial surface protein that polymerizes host actin filaments into comet tails, driving intracellular propulsion and direct spread between host cells without exposure to antibodies.

Listeria monocytogenes Microscopic Appearance

Short, regular gram positive rods or coccobacilli occurring singly, in pairs, or short chains resembling enterococci or pneumococci.

Listeria monocytogenes Cultural Morphology

Small, smooth, round, translucent colonies on sheep blood agar producing a narrow, subtle zone of complete beta hemolysis beneath the colony.

Listeria monocytogenes Temperature Range and Cold Enrichment

Wide psychrotrophic growth range from zero point five degrees to forty five degrees Celsius with optimum at thirty to thirty five degrees Celsius. Cold enrichment exploits growth at four degrees Celsius over weeks.

McBride Medium

Selective isolation medium containing lithium chloride and phenylethyl alcohol used to isolate Listeria monocytogenes from contaminated specimens.

Listeria Clinical Specimens

Placental tissue, amniotic fluid, maternal blood, newborn cerebrospinal fluid (CSF), and blood cultures.

Listeria Tumbling Motility

Characteristic end over end tumbling motility observed on hanging drop or wet mount preparations from broth incubated at room temperature (twenty two to twenty five degrees Celsius). Nonmotile at thirty seven degrees Celsius.

Listeria Motility Agar Pattern

Forms an umbrella shaped or inverted Christmas tree pattern of turbidity beneath the surface in semisolid motility medium at twenty two to twenty five degrees Celsius.

Listeria monocytogenes CAMP Test

Positive CAMP reaction. Synergistic beta hemolysis with beta lysin producing Staphylococcus aureus produces a rectangular or shovel shaped zone of clearing.

CAMP Reaction Comparison: Listeria vs Streptococcus agalactiae

Streptococcus agalactiae produces an arrowhead shaped zone of enhanced hemolysis, whereas Listeria monocytogenes produces a block or rectangular shovel shaped zone.

Hippurate Hydrolysis Test Principle

Detects the enzymatic hydrolysis of sodium hippurate to benzoic acid and glycine by bacterial hippuricase.

Ninhydrin Reagent in Hippurate Test

Deaminates liberated glycine to form a deep purple colored complex known as Ruhemann purple within ten to fifteen minutes.

Hippurate Hydrolysis in Listeria monocytogenes

Positive, developing a deep purple color upon addition of ninhydrin reagent.

Bile Esculin Hydrolysis Test Principle

Assesses an organism ability to grow in forty percent bile salts and hydrolyze esculin to esculetin and glucose.

Bile Esculin Positive Reaction

Esculetin reacts with ferric ammonium citrate indicator to produce a dark brown to black phenolic precipitate throughout the slant.

Bile Esculin Hydrolysis in Listeria monocytogenes

Positive, demonstrated by rapid blackening of bile esculin agar medium.

Listeria monocytogenes Biochemical Summary

Catalase positive, tumbling motility positive at twenty five degrees, esculin hydrolysis positive, hippurate positive, Voges Proskauer positive, and hydrogen sulfide negative on TSI.

Erysipelothrix rhusiopathiae General Characteristics

Slender, microaerophilic, non spore forming, catalase negative, gram positive rod that forms hydrogen sulfide on triple sugar iron agar.

Erysipelothrix rhusiopathiae Microscopic Morphology

Pleomorphic gram positive rods appearing as thin, long, filamentous threads in older cultures or short rods forming V shapes resembling diphtheroids. Frequently stains gram variable.

Erysipeloid Definition

A localized occupational skin infection characterized by a well defined, painful, non suppurative violaceous or purplish red cutaneous lesion with raised margins.

Erysipeloid High Risk Populations

Butchers, abattoir workers, fish handlers, veterinarians, and poultry processors through direct contact with infected animal carcasses or contaminated marine products.

Erysipelothrix Systemic Complications

Untreated localized lesions can progress to bacteremia, septicemia, and destructive infective endocarditis with aortic valve involvement.

Diffuse Cutaneous Erysipelothrix Infection

Widespread necrotic cutaneous lesions with central clearing and systemic constitutional symptoms.

Erysipelothrix Specimen of Choice

Full thickness skin biopsy or tissue aspirate taken from the advancing inflamed margin of the erysipeloid lesion.

Erysipelothrix rhusiopathiae Key Biochemical Profile

Oxidase negative, catalase negative, urease negative, esculin negative, nonmotile, and strongly positive for hydrogen sulfide production.

Gelatin Stab Growth of Erysipelothrix

Produces a characteristic test tube brush or pipe cleaner pattern of lateral growth radiating along the stab line at twenty two degrees Celsius without gelatin liquefaction.

Hydrogen Sulfide Production in Erysipelothrix

The only gram positive non spore forming bacillus that produces hydrogen sulfide gas, forming a distinct black ferrous sulfide precipitate in SIM or TSI butt.

Hydrogen Sulfide Detection Mechanism in SIM Medium

Organism reduces sodium thiosulfate to colorless hydrogen sulfide gas, which reacts with ferric ammonium citrate indicator to form an insoluble black precipitate.

Erysipelothrix Antibiotic Susceptibility

Highly sensitive to penicillin, cephalosporins, erythromycin, and chloramphenicol. Inherently resistant to vancomycin, sulfonamides, and aminoglycosides.

Erysipelothrix Drug of Choice

Penicillin is the primary drug of choice for both localized erysipeloid and systemic bacteremia.

Diagnostic Value of Vancomycin Resistance in Erysipelothrix

Intrinsic resistance to vancomycin serves as a valuable clinical clue to separate Erysipelothrix rhusiopathiae from other gram positive rods.

Actinomycetes General Features

Gram positive aerobic or facultative bacilli characterized by elongated branching vegetative hyphae resembling fungal mycelia.

Actinomycetes Culture and Filamentous Growth

Forms delicate branching filaments that penetrate into the agar substrate and produce aerial hyphae extending above the surface.

Lactobacillus acidophilus Common Name

Doderlein bacillus, recognized as the predominant beneficial microflora of the healthy adult female vagina.

Lactobacillus acidophilus Catalase Reaction

Catalase negative.

Protective Role of Vaginal Lactobacillus

Ferments vaginal glycogen to produce lactic acid, maintaining an acidic vaginal pH between three point eight and four point five that inhibits pathogenic colonization.

Lactobacillus Buffering Capacity Against Semen

Maintains vaginal acidity to counteract and neutralize the temporary alkaline pH of male seminal fluid during sexual intercourse.

Lactobacillus Morphology and Motility

Long slender gram positive non spore forming bacilli or pleomorphic coccobacilli that are strictly nonmotile.

Tomato Juice Agar

Enriched acidic culture medium specifically formulated for the isolation and selective cultivation of Lactobacillus species.

Lactobacillus on Papanicolaou (PAP) Smears

Routinely visualized as long rod shaped commensals on PAP stained cervical cytology screening smears.

Lactobacillus casei

Non pathogenic lactic acid bacterium widely utilized in fermented probiotic beverages and functional foods to support gastrointestinal tract digestion.