8 Enteric Gram Negative Obligate Aerobic & Anaerobic Bacilli (34)

Question 1

Case
A 75-year-old nursing home resident with swallowing disorder developed empyema following pneumonia. Cultures of aspirated empyema pus grew alpha hemolytic streptococcus which belonged to Streptococcus milleri group and also an anaerobic Gram-negative, pleomorphic bacillus which appeared as slender rods and coccobacillary forms. The anaerobic bacterium was non-sporing, non-motile, and produced black colonies on blood agar medium. It was saccharolytic and produced beta-lactamase.

The anaerobic isolate from the empyema pus is most likely to be which one of the following?

1 Leptotrichia buccalis
2 Bacteroides fragilis
3 Porphyromonas gingivalis
4 Veillonella parvula
5 Prevotella melaninogenica

Answer 1

Prevotella melaninogenica

The listed bacteria are all Gram-negative anaerobes. Of these, Prevotella melaninogenica (previously known as Bacteroides melaninogenicus) is the only bacterium that possesses all characteristics of the anaerobic isolate from the empyema pus. This bacterium is one of the indigenous oral flora known to produce endogenous infections. This elderly patient with a swallowing problem developed pneumonia due to aspiration of normal oropharyngeal flora (aspiration pneumonia) and empyema as a complication.
Pr.melaninogenica can cause a variety of conditions like aspiration pneumonia, empyema, lung abscess, brain abscess, face and neck abscesses, as well as dental, sinus, and soft tissue infections. It is reported to be associated with osteomyelitis, though rarely. Often, mixed infection with other anaerobic or facultative anaerobic flora is observed. Some strains of Pr.melaninogenica possess putative virulence factors. Fimbriated strains with hemagglutinating property have been isolated from periodontal disease. Strains with hemolysin production and proteolytic activity have also been described.
More than 60% of Pr.melaninogenica strains from pulmonary and oropharyngeal infections are found to produce beta-lactamase. Conventional method of laboratory diagnosis of Prevotella infections depends on cultural isolation of the bacteria and identification by phenotypic tests. Commercial kits are available for identification based on such tests. Molecular methods based on polymerase chain reaction (PCR) have been used for characterization and identification of Pr.melaninogenica isolates.
Leptotrichia buccalis is a large Gram-negative bacillus with typical fusiform shape. It does not produce pigmented colonies and is not known to produce beta-lactamase. This bacterium is a potential pathogen in neutropenic patients.
Bacteroides fragilis is the most common Gram-negative pathogenic anaerobe. It belongs to normal flora of the intestine. It is pleomorphic and produces beta-lactamase; its capsular polysaccharide is an important virulence factor. B. fragilis can tolerate oxygen because it produces superoxide dismutase.
Porphyromonas gingivalis produces black-pigmented colonies. It is asaccharolytic and not known to produce beta-lactamase. It is considered the main periodontal pathogen. Extracellular proteases called gingipains contribute to the virulence of the bacterium.
Veillonella parvula is a small anaerobic Gram-negative coccus, a normal inhabitant of mouth, intestine, and genital tracts. It has been isolated from clinical specimens from pleuropulmonary infections, meningitis, and endocarditis. V. parvula discitis or vertebral osteomyelitis has been reported, though extremely rarely.

Question 2

If a bacterium produces alkaline reaction in the slant and butt of the triple sugar iron tube it is most likely

1 Escherichia
2 Pseudomonas
3 Enterobacter
4 Klebsiella
5 Salmonella

Answer 2

Pseudomonas

riple sugar iron medium contains ferrous sulfate and 3 sugars: lactose, glucose and fructose. The glucose is one-tenth the concentration of the other sugars. The medium is made such that it has a poorly oxygenated area at the bottom of the tube called the butt and an angled well oxygenated area on top of the tube called the slant. If lactose or sucrose is fermented a large amount of acid is produced which turns the phenol red indicator yellow both in the butt and the slant. Some organisms generate gases which produce bubbles in the butt. If lactose is not fermented but the small amounts of glucose is, the oxygen-deficient butt is yellow, but on the slant the acid is oxidized to CO2 and H2O by the organism and the slant is red (neutral or alkaline). If neither lactose or glucose is fermented, both the butt and slant is red. The slant can sometimes be a deeper red-purple (more alkaline) due to production of ammonia from the deamination of amino acids. If H2S is produced, the black color of ferrous sulfide is seen.
Triple sugar iron agar reactions:

Slant Butt Gas H2S Representative genera
Acid Acid + - Escherichia, Klebsiella, Enterobacter
Alkaline Acid - - Serratia, Shigella
Alkaline Acid + + Salmonella, Proteus
Alkaline Alkaline - - Pseudomonas

Question 3

Pseudomonas aeruginosa is different from other pseudomonads in that it produces:

1 Pyocyanin
2 Chlorophyll
3 Anthocyanin
4 Endotoxin
5 Exotoxin

Answer 3

Pyocyanin

Pseudomonas aeruginosa produces 2 pigments that are useful for clinical and laboratory diagnosis: 1. Pyocyanin, which can produce blue color in a wound, and 2. Pyoverdin, a blue green pigment that fluoresces under ultraviolet light. In laboratory tests, these pigments diffuse into the agar and impart a characteristic blue-green color that is used for identification. Chlorophyll and anthocyanin are plant pigments and exotoxins, and endotoxins are not used to distinguish between different species of Pseudomonas.

Question 4

Which of the following enteric bacilli produce oxidase?

1 Escherichia coli
2 Klebsiella oxytoca
3 Proteus mirabilis
4 Pseudomonas aeruginosa

Answer 4

Pseudomonas aeruginosa

Escherichia coli, Klebsiella oxytoca, and Proteus mirabilis are gram negative rods and are resident flora of intestine. This group of enteric bacilli does not produce oxidase.
Pseudomonas aeruginosa
is present as normal intestinal flora and skin flora. The characteristics of this organism are as follows:
Gram negative rod
Motile
Obligate aerobe
Oxidase positive
Produce Pyocyanin pigment
Grows at 37° C-42° C
Do not ferment carbohydrates

Question 5

The family Enterobacteriaceae includes all of the following, EXCEPT

1 Escherichia coli
2 Klebsiella oxytoca
3 Proteus mirabilis
4 Pseudomonas aeruginosa
5 Providencia rettgeri

Answer 5

Pseudomonas aeruginosa

Escherichia coli, Klebsiella oxytoca, Proteus mirabilis, and Providencia rettgeri are enteric bacilli that are part of the family Enterobacteriaceae.
Pseudomonas aeruginosa is an obligate aerobe that belongs to the family Pseudomonadineae but is grouped with enteric bacilli.

Question 6

Which of the following organisms causes glanders in horses which can be transmitted to humans

1 Capnocytophaga
2 Cardiobacterium
3 Chromobacteria
4 Pseudomonas pseudomallei
5 Pseudomonas mallei

Answer 6

Pseudomonas mallei

Pseudomonas mallei is a small, nonmotile, aerobic gram negative organism. Glanders is a disease of the horse which results in pulmonary infection, ulcerative lesions, and lymphatic thickening with nodules. When transmitted to humans it can cause skin ulcers and mucous membrane ulcers leading to lymphangitis and sepsis. Primary pneumonia can be caused as a result of inhalation of the organisms.
These infections can be treated using tetracycline in combination of aminoglycoside.

Question 7

Which of the following organisms is associated with severe periodontal disease in juveniles and has gliding motility?

1 Capnocytophaga
2 Cardiobacterium
3 Chromobacteria
4 Pseudomonas pseudomallei
5 Pseudomonas mallei

Answer 7

Capnocytophaga

is gram-negative fusiform, or filamentous, bacilli that require CO2 for aerobic growth. This species is normal flora of oral cavity and is associated with bacteremia in immunosuppressed patients, in addition to juvenile periodontosis. Chemotactic activity of polymorphonuclear cells can be altered by a substance produced by these organisms.
The drug of choice is penicillin.

Question 8

Which of the following is the Gram negative rod that is part of the normal flora of the upper respiratory tract?

1 Capnocytophaga
2 Cardiobacterium
3 Chromobacteria
4 Pseudomonas pseudomallei
5 Pseudomonas mallei

Answer 8

Cardiobacterium

Cardiobacterium, which is an indigenous flora of oral cavity and bowel, is associated with endocarditis. It is a pleomorphic gram negative rod. This facultative anaerobe grows slowly in blood culture media and should be observed for weeks for the growth. This organism is susceptible to penicillin, tetracycline, and aminoglycosides.

Question 9

Which of the following organisms can grow at 42° C and is associated with melioidosis in animals and humans

1 Capnocytophaga
2 Cardiobacterium
3 Chromobacteria
4 Pseudomonas pseudomallei
5 Pseudomonas mallei

Answer 9

Pseudomonas pseudomallei

Melioidosis is a glanders-like disease caused by Pseudomonas pseudomallei. Lethal endotoxins produced by this organism inhibit protein and DNA synthesis in macrophages. The organism can survive and proliferate in polymorphonuclear and mononuclear leukocytes.

Question 10

Which one of the species of the following genus produces a violet pigment and is associated with abscess, diarrhea, and sepsis?

1 Capnocytophaga
2 Cardiobacterium
3 Chromobacteria
4 Pseudomonas pseudomallei
5 Pseudomonas mallei

Answer 10

Chromobacteria

Chromobacteria is motile, gram-negative slightly curved rod. Chromobacterium violaceum produces a violet pigment when cultured in broth or on agar. It is associated with fatal infections in humans and recurrent infections in chronic granulomatous disease.

Question 11

Bacteroides is

1 Spore forming anaerobic gram negative bacilli
2 Spore forming anaerobic gram positive bacilli
3 Spore forming aerobic gram negative bacilli
4 Non spore forming anaerobic gram positive bacilli
5 Non spore forming anaerobic gram negative bacilli

Answer 11

Non spore forming anaerobic gram negative bacilli

The characteristics of the genus Bacteroides includes all of the following:
Pleomorphic
Gram negative bacilli
Non spore former
Anaerobe
Cell wall is surrounded by polysaccharide capsule
Resident flora of gastrointestinal tract

Question 12

Which of the following is the most common anaerobe isolated from soft tissue infections?

1 Bacteroides caccae
2 Bacteroides fragilis
3 Prevotella bivia
4 Porphyromonas asaccharolyticus
5 Fusobacterium nucleatum

Answer 12

Bacteroides fragilis

Bacteroides fragilis is the most common anaerobe isolated from soft tissue infections. It is implicated in pleuropulmonary, intra-abdominal, and genital infections. The source of infection is generally endogenous and is associated with abscess formation. The endogenous bacterial population can spread by trauma or disease from the mucosal surfaces to sterile tissue or fluids. Nature of infection is polymicrobic in the initial period but host’s defense system gets rid of oxygen dependent and avirulent organisms. Proliferation of virulent organism leads to destruction of host tissue.

Question 13

Which of the following is true of Bacteroides fragilis?

1 Consists of capsular polysaccharides
2 Lacks lipopolysaccharide
3 Does not produce superoxide dismutase
4 Can not survive in the presence of oxygen
5 Is universally susceptible to penicillin

Answer 13

Consists of capsular polysaccharides

Capsular polysaccharides contribute to virulence of Bacteroides fragilis. It is antiphagocytic and anticomplementary. Bacteroides fragilis produces superoxide dismutase and therefore can survive in the presence of oxygen. It consists of the lipopolysaccharide but lacks endotoxic activity.
Capsular polysaccharide contributes to virulence of Bacteroides fragilis. It is antiphagocytic and anticomplementary. Bacteroides fragilis produces β-lactamase and therefore is resistant to penicillin.

Question 14

Which of the following Fusobacterium species is isolated from human infection most often?

1 Fusobacterium naviforme
2 Fusobacterium necrophorum
3 Fusobacterium nucleatum
4 Fusobacterium mortiferum
5 Fusobacterium varium

Answer 14

Fusobacterium nucleatum

The Fusobacterium sp. commonly isolated from human infection is Fusobacterium nucleatum.
Fusobacterium necrophorum is more virulent and causes more serious infection.
The characteristic feature of Fusobacterium nucleatum includes the following:
Gram negative long slender rod with tapering end
Associated with head, neck and lower respiratory tract infection
Indigenous flora of the upper respiratory and intestinal tract

Question 15

Case
A 20-year-old man presents with a cough for a few days with purulent sputum and fever. He has a condition that was diagnosed with an exam of his sweat chloride values by the quantitative pilocarpine iontophoresis test. The culture of the sputum shows Gram-negative rods, a blue-green pigment, and a sweet odor.

What is the most likely pathogen?

1 Bordetella pertussis
2 Legionella pneumophila
3 Listeria monocytogenes
4 Pseudomonas aeruginosa
5 Streptococcus pneumoniae

Answer 15

Pseudomonas aeruginosa

Sweat chloride values are abnormal in cystic fibrosis, an autosomal recessive disorder caused by a mutation in chromosome 7, producing a defective CFTR protein. These patients are very susceptible to respiratory infection by Pseudomonas aeruginosa, a Gram-negative rod that produces blue-green pigment cultures and a sweet odor.
Bordetella pertussis is the causative organism of pertussis.
Legionella pneumophila causes Legionnaire’s disease, a form of pneumonia.
Listeriosis is caused by Listeria monocytogenes. It is a form of food poisoning.
Streptococcus pneumoniae causes a variety of infections including pneumonia, meningitis, brain abscess, and osteomyelitis.

Question 16

Which of the following enteric bacilli produce oxidase?

1 Escherichia coli
2 Klebsiella oxytoca
3 Proteus mirabilis
4 Pseudomonas aeruginosa

Answer 16

Pseudomonas aeruginosa

Escherichia coli, Klebsiella oxytoca, and Proteus mirabilis are gram negative rods and are resident flora of intestine. This group of enteric bacilli does not produce oxidase.
Pseudomonas aeruginosa is present as normal intestinal flora and skin flora. The characteristics of this organism are as follows:

Gram negative rod
Motile
Obligate aerobe
Oxidase positive
Produce Pyocyanin pigment
Grows at 37° C-42° C
Do not ferment carbohydrates

Question 17

Which of the following antibiotics can be used to treat infection caused by Pseudomonas maltophilia?

1 Aminoglycosides
2 Cephalosporins
3 Imipenem
4 Sulfamethoxazole-trimethoprim

Answer 17

Sulfamethoxazole-trimethoprim

Pseudomonas maltophilia is sensitive to sulfamethoxazole-trimethoprim. Infection caused by this organism can not be treated by aminoglycosides, cephalosporins or imipenem as it is resistant to these antibiotics. It is also resistant to penicillin and quinolones.

Question 18

This organism causes glanders in horses which can be transmitted to humans

1 Capnocytophaga
2 Cardiobacterium
3 Chromobacteria
4 Pseudomonas pseudomallei
5 Pseudomonas mallei

Answer 18

Pseudomonas mallei

Pseudomonas mallei is small, nonmotile, aerobic gram negative organism. Glanders is disease of horse which results in pulmonary infection, ulcerative lesions, and lymphatic thickening with nodules. When transmitted to human it can cause skin ulcer and mucous membrane ulcer leading to lymphangitis and sepsis. Primary pneumonia can be caused as a result of inhalation of the organisms.
These infections can be treated using tetracycline in combination of aminoglycoside.

Question 19

This organism is associated with severe periodontal disease in juveniles and has gliding motility

1 Capnocytophaga
2 Cardiobacterium
3 Chromobacteria
4 Pseudomonas pseudomallei
5 Pseudomonas mallei

Answer 19

Capnocytophaga

Capnocytophaga is gram negative fusiform or filamentous bacilli that require CO2 for aerobic growth. This species is normal flora of oral cavity and is associated with bacteremia in immunosuppressed patients in addition to juvenile periodontosis. Chemotactic activity of polymorphonuclear cells can be altered by a substance produced by these organisms.
The drug of choice is penicillin.

Question 20

Gram negative rod that is part of normal flora of upper respiratory tract is

1 Capnocytophaga
2 Cardiobacterium
3 Chromobacteria
4 Pseudomonas pseudomallei
5 Pseudomonas mallei

Answer 20

Cardiobacterium

Cardiobacterium which is an indigenous flora of oral cavity and bowel, is associated with endocarditis. It is pleomorphic gram negative rod. This facultative anaerobe grows slowly in blood culture media and should be observed for weeks for the growth. This organism is susceptible to penicillin, tetracycline and aminoglycosides.

Question 21

One of the species of this genus produces a violet pigment and is associated with abscess, diarrhea and sepsis

1 Capnocytophaga
2 Cardiobacterium
3 Chromobacteria
4 Pseudomonas pseudomallei
5 Pseudomonas mallei

Answer 21

Chromobacteria

Chromobacteria is motile, gram-negative slightly curved rod. Chromobacterium violaceum produces violet pigment when cultured in broth or on agar. It is associated with fatal infection in humans and recurrent infections in chronic granulomatous disease.

Question 22

What is Bacteroides?

1 Spore forming anaerobic gram negative bacilli
2 Spore forming anaerobic gram positive bacilli
3 Spore forming aerobic gram negative bacilli
4 Non spore forming anaerobic gram positive bacilli
5 Non spore forming anaerobic gram negative bacilli

Answer 22

Non spore forming anaerobic gram negative bacilli

The characteristics of the genus Bacteroides includes all of the following:

Pleomorphic
Gram negative bacilli
Non spore former
Anaerobe
Cell wall is surrounded by polysaccharide capsule
Resident flora of gastrointestinal tract

Question 23

Which of the following is the most common anaerobe isolated from soft tissue infections?

1 Bacteroides caccae
2 Bacteroides fragilis
3 Prevotella bivia
4 Porphyromonas assaccharolytica
5 Fusobacterium nucleatum

Answer 23

Bacteroides fragilis

Bacteroides fragilis is the most common anaerobe isolated from soft tissue infections. It is implicated in pleuropulmonary, intra-abdominal and genital infections. Source of infection is generally endogenous and is associated with abscess formation. The endogenous bacterial population can spread by trauma or disease from the mucosal surfaces to sterile tissue or fluids. Nature of infection is polymicrobic in the initial period but host’s defense system gets rid of oxygen dependent and avirulent organisms. Proliferation of virulent organism leads to destruction of host tissue.

Question 24

Which of the following is true of Bacteroides fragilis?

1 Consists of capsular polysaccharides
2 Lacks lipopolysaccharide
3 Does not produce superoxide dismutase
4 Can not survive in the presence of oxygen
5 Is universally susceptible to penicillin

Answer 24

Consists of capsular polysaccharides

Capsular polysaccharides contribute to virulence of Bacteroides fragilis. It is antiphagocytic and anticomplementary. Bacteroides fragilis produces superoxide dismutase and therefore can survive in presence of oxygen. It consists of the lipopolysaccharide but lacks endotoxic activity.
Capsular polysaccharide contributes to virulence of Bacteroides fragilis. It is antiphagocytic and anticomplementary. Bacteroides fragilis produces β-lactamase and therefore is resistant to penicillin.

Question 25

Which of the following Fusobacterium species is isolated from human infection most often?

1 Fusobacterium naviforme
2 Fusobacterium necrophorum
3 Fusobacterium nucleatum
4 Fusobacterium mortiferum
5 Fusobacterium varium

Answer 25

Fusobacterium nucleatum

The Fusobacterium sp. commonly isolated from human infection is Fusobacterium nucleatum.
Fusobacterium necrophorum is more virulent and causes more serious infection.
The characteristic feature of Fusobacterium nucleatum includes the following:
Gram negative long slender rod with tapering end
Associated with head, neck, and lower respiratory tract infection
Indigenous flora of the upper respiratory and intestinal tract

Question 26

Which of the following aerobic gram-negative motile rods is associated with opportunistic infections in burn victims and immuno-compromised hosts? It is consistently resistant to many antibiotics and produces a distinctive colored pigment

1 Escherichia coli
2 Pseudomonas aeruginosa
3 Shigella flexneri
4 Clostridium perfringens

Answer 26

Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative motile rod which produces a characteristic water-soluble yellow pigment. This organism is a common bacteria that causes opportunistic infections in immuno-compromised individuals and burn patients. This bacteria is also known to be the most consistently resistant to antibiotics.
Escherichia coli and Shigella flexneri are both Enterobacteriaceae and are not known for pigment production. Clostridium perfringens is also associated with wound infections and is the causative agent of “gas gangrene”.

Question 27

Which of the following anaerobic gram-negative rods are opportunistic pathogens which cause both localized abscesses as well as systemic infections and bacteremia?

1 Shigella flexneri
2 Clostridium perfringens
3 Bacteroides fragilis
4 Escherichia coli

Answer 27

Bacteroides fragilis

Bacteroides fragilis is commonly associated with abscess formation and produces an anti-phagocytic capsule. This organism is anaerobic but can tolerate some atmospheric oxygen, forming colonies overnight on blood agar. These bacteria may be resistant to tetracycline but are generally susceptible to chloramphenicol and other antibiotics. Resistance may be plasmid borne in these organisms.
Escherichia coli and Shigella flexneri are both Enterobacteriaceae which are not commonly associated with the formation of abscesses. Clostridium perfringens causes “gas gangrene”.

Question 28

Case
A 6-year-old boy presents with fever and cough. He has history of several episodes of pneumonia. A sweat test reveals an increased amount of chloride. He is coughing up thick, greenish sputum. Temperature is 37.6°C. A Gram stain of the sputum reveals gram-negative rods and a culture grows a gram-negative rod that is oxidase-positive and produces a blue-green pigment.

What is the most likely cause of the infection?

1 Pseudomonas aeruginosa
2 Haemophilus influenzae
3 Bordetella pertussis
4 Legionella pneumophila
5 Streptococcus pneumoniae

Answer 28

Pseudomonas aeruginosa

Pseudomonas aeruginosa is a gram-negative oxidase-positive rod that does not ferment glucose, with a typical metallic sheen of the growth on TSI agar, coupled with the blue-green pigment on ordinary nutrient agar and a fruity aroma.
P. aeruginosa produces 2 pigments:
pyocyanin which can color the pus in a wound blue
pyoverdin (fluorescein), a yellow-green pigment that fluoresces under ultraviolet light
P. aeruginosa is the only species of Pseudomonads that synthesizes pyocyanin. Pseudomonads are able to grow in water containing only traces of nutrients, and they have a remarkable ability to withstand disinfectants. Strains of P. aeruginosa isolated from cystic fibrosis patients have a prominent slime layer (glycocalyx), which gives their colonies a very mucoid appearance.
P. aeruginosa is primarily an opportunistic pathogen that causes infections in
hospitalized patients
those with chronic respiratory disease in whom the normal clearance mechanisms are impaired
immunosuppressed patients with neutrophil counts of less than 500/μL
patients with indwelling catheters
P. aeruginosa can cause infections virtually anywhere in the body, but urinary tract infections, pneumonia (especially in cystic fibrosis patients), and wound infections (especially burns) predominate.
Haemophilus influenzae is a small gram-negative rod with a polysaccharide capsule. It is the leading cause of meningitis in young children, and it is an important cause of upper respiratory tract infections and sepsis in children; it causes pneumonia in adults, particularly those with chronic obstructive lung disease.
Bordetella pertussis is a small coccobacillary encapsulated gram-negative rod. It causes whooping cough (pertussis).
Legionella pneumophila is gram-negative rod that stains faintly with the standard Gram stain. Sputum Gram stains reveal many neutrophils, but no bacteria. The organism fails to grow on ordinary media in a culture of blood or sputum, but it will grow on medium supplemented with iron and cysteine. L. pneumophila causes atypical pneumonia, both in the community and in hospitalized immunocompromised patients.
Streptococcus pneumoniae causes pneumonia, bacteremia, meningitis, and infections of the upper respiratory tract. Pneumococci are gram-positive, lancet-shaped cocci arranged in pairs or short chains.

Question 29

Case
A 28-year-old woman was examined on the surgical unit. She was previously admitted for an appendectomy. The patient’s clinical history is significant for spiking temperatures within the last 24 hours of between 38.9° C - 40.1° C. An examination of the surgical site reveals swelling and an erythematous area extending outward from the site. There is midgastric and right lower quadrant abdominal tenderness. There is some noticeable drainage present on one end. The drainage is collected in a sterile syringe and sent to the laboratory for culture and gram stain. Blood cultures are also obtained.
The gram stain is significant for the presence of light staining gram-negative rods. After 48 hours, the culture is significant for a gram-negative anaerobic rod that grows in 20% bile; is catalase and indole positive; resistant to kanamycin, vancomycin, and colistin. The blood cultures eventually are also positive for the same organism.

Based on the history of the patient and the preliminary culture findings, what is the most probable organism causing this intra-abdominal infection?

1 Clostridium perfringens
2 Bacteroides fragilis
3 Fusobacterium spp
4 Veillonella spp
5 Propionibacterium acnes

Answer 29

Bacteroides fragilis

Bacteroides fragilis are anaerobic, gram-negative bacilli that grow in 20% bile; they are resistant to kanamycin, vancomycin, and colistin and are catalase and indole positive. It is the most common anaerobic isolate from intra-abdominal abscesses. The organism is part of the normal gastrointestinal flora. Anaerobic infections are usually polymicrobial with a mixture of aerobes and anaerobes. Metronidazole, clindamycin, and cefoxitin are some of the antibiotics used to treat Bacteroides fragilis.
Clostridium perfringens is an anaerobic, gram-positive, spore-forming bacillus. The organism is nonmotile and has a distinctive “box car” appearance on gram stain. It produces oval central spores, but they are rarely seen in clinical or cultures. All types produce lecithinase, which is lytic to neutrophils. Because of the action of lecithinase on neutrophils, gram stains of blister aspirates and other drainage characteristically contain gram-positive rods but little or no neutrophils. Clostridium perfringens is the main causative agent of gas gangrene, and diagnosis is made by microbiological findings, clinical findings, and the demonstration of myonecrosis at surgery. Surgical, antibiotic, and hyperbaric oxygen treatments are used in treating gas gangrene.
Fusobacterium spp. are long, thin anaerobic gram-negative rods with pointed ends. They are usually arranged in end-to-end pairs. The organism is indole variable, catalase negative, grows in 20% bile, and is sensitive to kanamycin and colistin but resistant to vancomycin. The organism is a cause of brain abscesses, sinusitis, odontogenic infections, pleuropulmonary infections, bacteremia, and endocarditis. Most Fusobacterium spp. are sensitive to penicillin.
Veillonella spp. is anaerobic gram-negative cocci that are found as part of the normal flora in the oral cavity, upper respiratory tract, intestine, and vagina. They have a generally low virulence but can cause significantly serious infections given the right circumstances, such as steroid therapy, previous injury, foreign bodies, and IV drug abuse. They are generally susceptible to most of the antibiotics used to treat other anaerobic infections.
Propionibacterium acnes is a gram-positive anaerobic cocci. It is part of the normal skin flora. Though the organism is usually a contaminant of blood and wound cultures, it can be the cause of serious life-threatening infections in the proper conditions. These infections include brain abscesses, osteomyelitis, subdural empyema, cerebral shunt infection, infective endocarditis, and parotid and dental infections.

Question 30

Case
A 25-year-old woman presents with severe orbital pain. She claims to have difficulty smelling and tasting. This symptoms have been present for 2 weeks. An examination of her throat is significant for the presence of sinus drainage. There is no redness of the throat area. The ears are clear. The patient has a body temperature of 38.6° C and an elevated WBC with a left shift. Sinus material was obtained for culture and gram stain. The gram stain was positive for numerous neutrophils with no organisms seen. On day 2, the culture was positive for 4+ gram negative rods that were sensitive to kanamycin and colistin, resistant to vancomycin, and catalase negative. Morphologically, the organism Gram-stained as long and pointy rods.

What organism is most likely causing the infection?

1 Clostridium perfringens
2 Bacteroides fragilis
3 Fusobacterium spp
4 Veillonella spp
5 Propionibacterium acnes

Answer 30

Fusobacterium spp

Fusobacterium spp. are long, thin anaerobic Gram-negative rods with pointed ends. They are usually arranged in end-to-end pairs. The organism is indole variable, catalase negative, grows in 20% bile, and is sensitive to kanamycin and colistin but resistant to vancomycin. The organism is a cause of brain abscesses, sinusitis, odontogenic infections, pleuropulmonary infections, bacteremia, and endocarditis. Most Fusobacterium spp. are sensitive to penicillin.
Bacteroides fragilis are anaerobic, Gram-negative bacilli that grow in 20% bile; they are resistant to kanamycin, vancomycin, and colistin and are catalase and indole positive. It is the most common anaerobic isolate from intra-abdominal abscesses. The organism is part of the normal gastrointestinal flora. Anaerobic infections are usually polymicrobial with a mixture of aerobes and anaerobes. Metronidazole, clindamycin, and cefoxitin are some of the antibiotics used to treat Bacteroides fragilis.
Clostridium perfringens is an anaerobic, Gram-positive, spore-forming bacillus. The organism is nonmotile and has a distinctive “box car” appearance on gram stain. It produces oval central spores, but they are rarely seen in clinical or cultures. All types produce lecithinase, which is lytic to neutrophils. Because of the action of lecithinase on neutrophils, gram stains of blister aspirates and other drainage characteristically contain gram-positive rods but little or no neutrophils. Clostridium perfringens is the main causative agent of gas gangrene, and diagnosis is made by microbiological findings, clinical findings, and the demonstration of myonecrosis at surgery. Surgical, antibiotic, and hyperbaric oxygen treatments are used in treating gas gangrene.
Veillonella spp. is anaerobic Gram-negative cocci that are found as part of the normal flora in the oral cavity, upper respiratory tract, intestine, and vagina. They have a generally low virulence but can cause significantly serious infections given the right circumstances, such as steroid therapy, previous injury, foreign bodies, and IV drug abuse. They are generally susceptible to most of the antibiotics used to treat other anaerobic infections.
Propionibacterium acnes is a Gram-positive anaerobic cocci. It is part of the normal skin flora. Though the organism is usually a contaminant of blood and wound cultures, it can be the cause of serious life-threatening infections in the proper conditions. These infections include brain abscesses, osteomyelitis, subdural empyema, cerebral shunt infection, infective endocarditis, and parotid and dental infections.

Question 31

Case
A 37-year-old Caucasian woman swims regularly for exercise. She is diabetic but is otherwise in good health. She swims 100 laps, 4 to 6 times per week. She starts to notice severe right ear pain. She also notes that her right ear is very itchy. She sees her family doctor and mentions her complaints. When he goes to insert the otoscope, he gently pulls on her ear. This causes her quite a bit of pain. He notes an inflamed external ear canal, but the tympanic membrane is normal.

What is the classification of the most likely pathogen?

1 Gram-positive aerobic rod
2 Gram-negative aerobic rod
3 Gram-negative anaerobic rod
4 Gram-negative cocci
5 Gram-positive cocci

Answer 31

Gram-negative aerobic rod

This patient has signs and symptoms of otitis externa, specifically, “swimmers ear”. An infection of the external auditory canal, usually due to bacteria, is called otitis externa. Symptoms include ear pain and itching. The finding of an inflamed external ear canal is consistent with otitis externa.
The most likely pathogen for swimmer’s ear is Pseudomonas aeruginosa, especially in patients with diabetes, who are particularly prone to infection with Pseudomonas. Pseudomonas aeruginosa is a Gram-negative aerobic rod. Other pathogens that can cause otitis externa include Peptostreptococcus, Staphylococcus aureus, Bacteroides, Proteus, and fungi.

Question 32

Case
A 37-year-old Caucasian woman swims regularly for exercise. She swims 100 laps 4-6 times per week. She starts to notice severe right ear pain. She also notes that her right ear is very itchy. She sees her family doctor and mentions her symptoms. When the doctor goes to insert the otoscope, they gently pull on her ear. This causes her quite a bit of pain. The doctor notes an inflamed external ear canal, but the tympanic membrane is normal.

What is the most likely pathogen?

1 Pseudomonas aeruginosa
2 Streptococcus pneumoniae
3 Haemophilus influenzae
4 Branhamella catarrhalis
5 Corynebacterium diphtheriae

Answer 32

Pseudomonas aeruginosa

his patient has signs and symptoms of otitis externa, specifically “swimmer’s ear.” An infection of the external auditory canal, usually due to bacteria, is called otitis externa. Symptoms include ear pain and itching. The finding of an inflamed external ear canal is consistent with otitis externa. The most likely pathogen for swimmer’s ear is Pseudomonas aeruginosa (Refer to the image). Pseudomonas aeruginosa is a gram-negative aerobic rod. Other pathogens that can cause otitis externa include Peptostreptococcus, Staphylococcus aureus, Bacteroides, Proteus, and fungi.
Streptococcus pneumoniae, Haemophilus influenzae, and Branhamella catarrhalis are all causes of otitis media, or inner ear infection. Corynebacterium diphtheriae does not infect the ear.

Question 33

A 22-year-old man recovering from third-degree burns develops a wound infection. There is a distinct grape-smelling odor and a blue pigment develops when grown in vitro on nutrient agar. What is the most likely etiology?

1 Acinetobacter
2 Aeromonas
3 Moraxella
4 Pseudomonas
5 Plesiomonas

Answer 33

Pseudomonas

he most likely diagnosis is Pseudomonas aeruginosa. Pseudomonas is an oxidative Gram-negative rod. It produces a characteristic blue pigment when grown on agar. Wounds infected with Pseudomonas also have a characteristic grape-smelling odor. Pseudomonas can survive in water. It is an opportunistic infection that commonly affects immunocompromised patients. It is particularly dangerous for patients with cystic fibrosis. It is also a common cause of infections in hospitalized patients.
Acinetobacter is a Gram-negative coccobacillary organism. It also survives well in wet environments. It most commonly causes pneumonia or urinary tract infections. In hospitals, it may be transmitted through contaminated respiratory equipment. It does not produce pigment or have a characteristic smell.
Aeromonas is a rare cause of soft tissue infections acquired through fresh or salt water. Although uncommon, it is highly virulent. It does not produce pigment or have a characteristic smell.
Moraxella is a Gram-negative rod most commonly associated with respiratory tract infections. It does not produce pigment or have a characteristic smell.
Pleisomonas is a Gram-negative rod that may produce an enterotoxin-mediated diarrhea. It does not produce pigment or have a characteristic smell.