Anaerobes of Clinical Importance: Specimen Processing, Cultivation, and Identification

Comprehensive practice flashcards covering clinical anaerobic bacteriology, including atmospheric requirements, endogenous normal flora, pathogenesis, clinical disease manifestations, specimen collection criteria, selective media, incubation systems, identification procedures, and therapeutic approaches.

What is the defining biological characteristic of an obligate anaerobe?

Obligate anaerobes are strict anaerobes that are killed almost immediately in the presence of oxygen and require oxygen-free growth conditions in the laboratory.

How do facultative anaerobes differ from aerotolerant anaerobes in their atmospheric requirements and metabolism?

Facultative anaerobes prefer to use oxygen but can grow at a slower rate in its absence; aerotolerant anaerobes can survive a short exposure to oxygen but cannot perform metabolic processes during that exposure.

What is the primary difference between endogenous and exogenous anaerobic microbiota?

Endogenous microbiota already exist within the human body on mucosal surfaces and skin, whereas exogenous microbiota exist outside the body (such as in soil or sediment) and gain entry via contact, ingestion, or inhalation.

Which two bacterial genera represent the predominant indigenous anaerobic members colonizing human skin?

Cutibacterium (specifically Cutibacterium acnes, Gram-positive rods colonizing sebaceous glands and hair follicles) and Peptostreptococcus (Gram-positive cocci).

What percentage of the normal microbial flora residing in saliva, nasal washings, and tooth scrapings consists of anaerobes?

$90\%$ of the bacteria in saliva, nasal washings, and gingival and tooth scrapings are anaerobes.

Which three anaerobic Gram-negative rod genera represent the most common normal flora of the human respiratory tract?

Prevotella, Porphyromonas, and Fusobacterium.

Approximately how many bacterial species inhabit the human gastrointestinal tract, and which anaerobic group is most predominant?

Approximately $500\text{--}1000$ different species live in the GI tract, with members of the Bacteroides fragilis group being the most common.

What proportion of bacteria present in normal cervical and vaginal secretions are anaerobes?

$50\%$ of bacteria in cervical and vaginal secretions are anaerobes.

What are common predisposing factors that lead to clinical anaerobic infections?

Trauma to protective skin/mucosal barriers, vascular stasis, human or animal bite wounds, oral surgery, gastrointestinal surgery or bowel obstruction, genital tract surgery, and the introduction of soil into open wounds.

Based on this endospore morphology comparison, which clinically significant Clostridium species is shown in Panel A forming round terminal spores?

Clostridium tetani, which forms characteristic terminal spores located at the end of the vegetative cell (giving a drumstick appearance).

What are three hallmark physical and clinical indicators suggesting anaerobic involvement in a patient's infection?

The presence of a foul odor, the accumulation of a large quantity of gas, and the presence of sulfur granules (dense bacterial clumps).

How do Type A and Type C food poisoning caused by Clostridium perfringens differ in incubation period and severity?

Type A has an incubation of $8\text{--}30\text{ hours}$ and causes a mild, self-limiting diarrhea lasting $24\text{ hours}$; Type C has an incubation of $5\text{--}6\text{ hours}$ and causes severe abdominal pain, vomiting, bloody diarrhea, and potentially bowel perforation, with a $15\text{--}25\%$ fatality rate.

What is the mechanism of foodborne botulism caused by Clostridium botulinum, and what are common dietary sources?

It is caused by the ingestion of preformed botulinum neurotoxin in preserved foods, such as home-canned vegetables, home-cured meats (ham), and fermented fish.

What clinical therapeutic and cosmetic applications exist for botulinum toxin A?

Botulinum toxin A (Botox) is used medically to treat migraines and cosmetically to remove wrinkles.

What are the classic symptoms and time to onset of foodborne botulism after ingestion?

Symptoms appear $2\text{ hours to } 8\text{ days}$ after ingestion and include weakness, paralysis, double or blurred vision, impaired speech, and difficulty in swallowing.

What neurotoxin mediates the clinical manifestations of tetanus, and what key symptoms does it induce?

Tetanospasmin, which causes spastic paralysis resulting in lockjaw, a distorted grin, and difficulty breathing.

What is the incubation period for Clostridium tetani following a puncture wound?

Symptoms appear $7\text{--}21\text{ days}$ after injury, with the length of the incubation period depending on the wound's distance from the central nervous system.

Which organism is the most common cause of myonecrosis (gas gangrene), and what clinical signs develop in the wound?

Clostridium perfringens; clinical signs include severe pain, swelling, fluid-filled blisters, foul discharge, skin discoloration, and extensive tissue necrosis.

What percentage of healthy individuals carry Clostridium difficile as normal gastrointestinal flora?

Clostridium difficile is present as part of the normal gastrointestinal flora in approximately $5\%$ of healthy individuals.

How does antimicrobial therapy trigger Clostridium difficile-associated infection and diarrhea?

Broad-spectrum antimicrobials destroy the normal intestinal flora, enabling resistant C. difficile to proliferate unchecked and produce high levels of toxin A and toxin B.

Into which two phyla are Gram-positive, non-spore-forming anaerobic bacilli divided?

Actinobacteria (including Actinomyces, Bifidobacterium, Eggerthella, Mobiluncus, and Cutibacterium) and Firmicutes (including Lactobacillus).

What is actinomycosis, and what characteristic macroscopic structure is found in its purulent drainage?

A chronic granulomatous disease characterized by sinus tracts and fistulae that produce purulent drainage containing yellowish clumps called sulfur granules.

What microbiological change in the vaginal ecosystem leads to bacterial vaginosis (BV)?

A loss of normal Lactobacillus flora accompanied by an overgrowth of endogenous anaerobes, resulting in a homogenous, malodorous gray-white discharge.

Which anaerobic Gram-positive coccus is considered the most pathogenic and is frequently isolated in pure culture?

Finegoldia magna (often implicated in endocarditis, meningitis, and pneumonia).

Which genus of anaerobic Gram-negative cocci is consistently regarded as pathogenic when isolated in mixed culture from abscesses?

Veillonella.

What are acceptable specimen types and collection methods for diagnosing central nervous system anaerobic infections?

Cerebrospinal fluid ($\text{CSF}$), needle-aspirated abscess material, and tissue collected during biopsy or autopsy.

What is the proper method for collecting an acceptable anaerobic specimen from a localized closed abscess?

Needle and syringe aspiration of the closed abscess.

What is the only acceptable method for collecting a urine specimen intended for anaerobic culture?

Suprapubic bladder aspiration (voided or catheterized urine must be rejected).

Which respiratory and superficial specimens must be rejected as unsuitable for anaerobic culture?

Expectorated sputum, swabs of superficial skin ulcers, surface abscess swabs, foot ulcer swabs, and exposed wound swabs.

Under what specific clinical circumstances are swabs considered acceptable for anaerobic specimen collection?

Only when aspiration is impossible or when a biopsy specimen cannot be obtained, provided they are placed in dedicated anaerobic swab transport systems.

What critical handling steps must be performed after aspirating pus with a needle and syringe?

Any air in the syringe must be expelled immediately, the needle must ALWAYS be removed before transport, and $2\text{--}3\text{ drops}$ of aspirate should be used per agar plate.

How should deep tissue and bone specimens be processed prior to inoculating anaerobic culture media?

The tissue or bone is combined with liquid media inside a tissue grinder and homogenized until a thick suspension is formed.

What five standard laboratory procedures should be performed on specimens submitted for anaerobic culture?

Macroscopic examination, direct microscopic examination (Gram stain), preparation of smears, inoculation of selective and nonselective media, and anaerobic incubation.

What diagnostic information is provided by finding squamous epithelial cells during direct microscopic examination of an anaerobic specimen?

It indicates that the specimen was contaminated with normal microbiota from an adjacent mucosal surface during collection.

Which three nutritional supplements are required by fastidious anaerobic bacteria in culture media?

Vitamin K, hemin, and yeast extract.

What is the composition and purpose of CDC anaerobic blood agar (AnBAP)?

Trypticase soy agar with $5\%$ sheep blood, supplemented with yeast extract, hemin, vitamin K, and L-cysteine; it is an enriched, nonselective primary medium that supports all obligate and facultative anaerobes.

What is the mechanism of selectivity of anaerobic phenylethyl alcohol blood agar (PEA)?

Phenylethyl alcohol inhibits the swarming of Proteus and suppresses facultative Gram-negative rods (Enterobacteriaceae) while supporting obligate anaerobes and Gram-positive facultative anaerobes.

What is the selective function of vancomycin and kanamycin in anaerobic KV blood agar?

Vancomycin inhibits Gram-positive obligate and facultative anaerobes, while kanamycin inhibits most facultative Gram-negative rods, thereby selecting for Gram-negative obligate anaerobes.

What does 'laked blood' refer to in paromomycin-vancomycin laked blood agar (PVLB), and what is its diagnostic advantage?

Blood that has undergone multiple freeze-thaw cycles; laking lyses the erythrocytes, allowing brown/black pigment production by Prevotella species to be detected much earlier.

Where do obligate aerobes, microaerophiles, and obligate anaerobes locate their growth in an enriched thioglycolate broth tube?

Obligate aerobes and microaerophiles grow near the oxygenated top of the broth, obligate anaerobes grow near the reduced bottom, and facultative organisms grow throughout.

What biochemical activity is demonstrated by the dark brown-black discoloration on this Bacteroides bile-esculin (BBE) agar plate?

The organism can grow in the presence of $20\%$ bile and hydrolyzes esculin to produce an insoluble brown-black complex, characteristic of the Bacteroides fragilis group.

What organism forms yellow, rhizoid colonies with speckled opalescence on cycloserine-cefoxitin fructose agar (CCFA), turning the agar yellow?

Clostridioides difficile.

What is the minimum recommended battery of media required for processing clinical anaerobic cultures?

A nonselective anaerobic blood agar (CDC AnBAP or Brucella), a biplate containing BBE and LKV, a PEA or CNA plate, and an enriched thioglycolate broth tube.

What specialized media packaging system is shown here, designed to prevent oxygen exposure during transit and storage?

Prereduced, anaerobically sterilized ($\text{PRAS}$) plated media, which are manufactured, packaged, shipped, and stored entirely under anaerobic conditions in foil pouches.

What gas mixture is maintained within a standard anaerobic chamber?

$5\%$ hydrogen ($\text{H}_2$), $5\text{--}10\%$ carbon dioxide ($\text{CO}_2$), and $85\text{--}90\%$ nitrogen ($\text{N}_2$).

What roles do the catalyst and the desiccant play within an anaerobic chamber?

The palladium catalyst removes residual oxygen by combining it with hydrogen to form water, and the desiccant absorbs the resulting water vapor.

In this anaerobic GasPak jar system, what chemical reaction occurs when water is added to the envelope, and how long does it take to achieve anaerobiosis?

Water reacts with sodium bicarbonate and sodium borohydride to release $\text{CO}_2$ and $\text{H}_2$, which react over a palladium catalyst to remove oxygen, achieving anaerobiosis in approximately $30\text{--}45\text{ minutes}$.

When should anaerobic incubation plates first be examined, and what is the total incubation holding time?

Plates should first be examined after $48\text{ hours}$ of incubation and held for a total of $5\text{--}7\text{ days}$ to allow slow-growing anaerobes to develop.

How is the aerotolerance test performed to differentiate strict anaerobes from facultative anaerobes?

The isolated colony is subcultured to two agar plates: one incubated anaerobically and one incubated in $\text{CO}_2$ at $35^\circ\text{C}$ for $48\text{ hours}$; obligate anaerobes grow exclusively on the anaerobic plate.

Under long-wave ultraviolet light, which anaerobic organisms produce the brick red fluorescence (A) and chartreuse fluorescence (B) shown here?

Plate A shows Porphyromonas asaccharolytica (brick red fluorescence), and Plate B shows Fusobacterium nucleatum (chartreuse fluorescence).

How does the catalase test shown here differentiate aerotolerant Clostridium strains from Bacillus species?

Aerotolerant Clostridium species are catalase negative (no bubbling), whereas Bacillus species are catalase positive (vigorous bubbling).

What is the diagnostic significance of a positive spot indole reaction (blue color, as shown on the left) when evaluating Gram-positive coryneform anaerobes?

It presumptively identifies Cutibacterium acnes, which is spot indole positive, differentiating it from other Cutibacterium species.

Which single Clostridium species produces the rapid positive (bright pink) urease reaction shown in the right tube?

Clostridium sordellii.

What are the susceptibility patterns of Bacteroides fragilis, Fusobacterium, and Clostridium perfringens to special potency kanamycin, vancomycin, and colistin disks?

B. fragilis is resistant to all three disks; Fusobacterium is resistant to vancomycin only; C. perfringens is resistant to colistin only.

Which anaerobic Gram-positive coccus is presumptively identified by zone inhibition around a sodium polyanethol sulfonate (SPS) disk?

Peptostreptococcus anaerobius is SPS-sensitive, whereas other anaerobic Gram-positive cocci are resistant.

How are lecithinase, lipase, and proteolytic enzymatic reactions differentiated on egg yolk agar (EYA)?

Lecithinase produces an opaque halo around colonies; lipase produces an iridescent multicolored sheen ('gasoline on water'); proteolytic enzymes produce a completely clear zone of clearing.

What anaerobic organism produces the target-like double zone of beta-hemolysis shown on this blood agar plate?

Clostridium perfringens.

Which Gram-positive, non-spore-forming anaerobic bacillus displays the delicate, branching, beaded filamentous morphology shown in this Gram stain?

Actinomyces species.

Which bile-resistant, catalase-positive, and nitrate-positive anaerobic Gram-negative rod forms characteristic 'fish-eye' colonies on BBE agar as shown?

Bilophila wadsworthia.

What organism displays long, slender Gram-negative bacilli with sharply tapered, pointed ends (fusiform rods) as seen in this Gram stain?

Fusobacterium nucleatum.