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Case sixteen: a common organism in an uncommon place.

The next day, the blood culture was reported as positive with Gram-negative bacilli. The patient was admitted for the management of bacteremia and meningitis. The patient's CSF studies showed low glucose and high protein levels, consistent with bacterial meningitis (see Table 2). However, there were no organisms seen on Gram-stained smear, despite the presence of leukocytes. CSF cultures were negative. The patient was treated with ceftriaxone and ampicillin. Careful review of the history revealed no potential direct animal exposures, although there were several household outdoor dogs. In addition, there was no history of travel.

The patient's hospital course was complicated by persistent fevers and development of subdural effusions that were confirmed on radiographic imaging of head. These effusions required serial lumbar taps for relief.

In the laboratory, the Gram-stained smear of the positive blood culture bottle exhibited Gram-negative coccobacilli. The blood culture bottle was subcultured to chocolate and 5% sheep blood and MacConkey agar. The media incubated at 35[degrees] C in 5% CO2 for 24 hours. Following incubation, multiple 0.5 cm diameter, grey, non-hemolytic colonies were noted to have grown on the chocolate and 5% sheep blood agar plates (Figure 1). The organism was subsequently identified, using the Remel Rapid NF testing kit for glucose-nonfermenting Gram-negative bacteria. Additional characteristics included: facultative anaerobe, positive oxidase and catalase tests. The spot indole test was also positive and confirmatory of the identification.



Pasteurella spp. are normal flora of the respiratory and gastrointestinal tracts of many species of domestic and wild animals. They are etiological agents of hemorrhagic septicemia, gastrointestinal infections, and respiratory tract infections in many animals. The current taxonomy of this genus has recently changed with some members transferred to the genus Aggregatibacter and several new species being added. Currently there are 21 species listed within this genus.

Humans acquire Pasteurella primarily through animal exposure. Most human infection are associated with animal bites, but respiratory tract infections can be caused by inhalation of the organism. Pasteurella multocida includes three subspecies, multocida, septica, and gallicida. It is the most commonly isolated human pathogen from bite wounds but has been be recovered from respiratory secretions from patients with pneumonia, cerebrospinal fluid, abscesses, biopsy specimens from patients with osteomyelitis, blood, joint fluid from septic arthritis, and many other sites. Other species of Pasteurella are more often associated with animal disease, although they have been reported from human cases of endocarditis, pneumonia, septicemia, and wounds, occasionally involving osteomyelitis. The biochemical reactions of the most common species of Pasteurella are listed in Table 1. For a more complete discussion of the taxonomy of Pasteurella spp., see ref. 1.

Animal Reservoir

P. multocida subsp. multocida is routinely isolated from the respiratory tract of household cats and dogs and from human infections after bites or close proximity to pets. It is also the most clinically significant Pasteurella subspecies. The frequency of Pasteurella multocida carriage varies by the animal species. Cats and dogs have the highest rates of colonization, 70-90% and 15-55%, respectively. Most animals carry the organism asymptomatically, although P. multocida can cause sporadic and epidemic illness in animals, such as pneumonia and septicemia in cattle, sheep and swine, fowl cholera in chickens, turkeys and ducks. P. multocida subspecies septica and gallicida are also similar in distribution, although less commonly found as human pathogens. Identification to the subspecies level is of greatest clinical significance to veterinary medicine. P. multocida is also widely cultured from other animals, such as domesticated and wild, e.g., lions, panthers, and various birds. They have been attributed to atrophic rhinitis in pigs and "shipping fever" in cattle, when the animals were under stress and in close proximity while being herded across long distance.

Clinical Presentation

Humans do not usually harbor the organism. The majority of human infection is cellulitis and wound infection (direct contact with animal via licking or biting of hands or feet). Besides local infections, P. multocida is capable of causing systemic diseases such as meningitis, septicemia, peritonitis, arthritis, endocarditis, and osteomyelitis. Infections have also been reported with underlying medical conditions such as diabetes, steroid therapy, and immunosuppression. In one literature review, up to 68% of cases of P. multocida meningitis occurred after skull fracture or neurosurgery.

There have been case reports of neonates in close proximity to pets (cats and dogs) becoming infected with P. multocida. The routes of transmission often are ascribed to pets licking the fingers or faces of a young child, without a history of bites or trauma. In this case, the mother did not report a history of bites, although a clear history of a household pet was elicited.

Central nervous system infections by P. multocida has been well-characterized in the elderly, immunocompromised, and young children and infants. Examples include meningitis, subdural empyema, and brain abscess. CSF typically shows bacterial infection, with elevated WBC's (neutrophils dominant), low glucose (<2.2 mmol/L), no RBC's (non-traumatic tap), and increased protein (>0.45 g/L) Meningitis is an infection predominantly involving the subarachnoid space, but can refer to focal or generalized involvement of the brain tissue. Meningitis due to Gram-negative bacilli can also be a complication of neurosurgical procedures such as craniotomy.


Virulence of Pasteurella is related to the polysaccharide capsule that allows the organism to resist phagocytosis. Although some strains produce a cytotoxin, its role in the pathogenesis of disease is not clear. P. multocida appears to be the most virulent of the species.Virulence may be enhanced by the ability of the organism to utilize free iron, a factor in the pathogenicity of other bacterial species.

Laboratory Identification

Pasteurella spp. grow well on routine laboratory media, including blood agar. Growth is not enhanced by C[O.sub.2]. Colonies are small and translucent and may be smooth or rough. A brown discoloration of the medium may develop around colonies. The organisms on blood agar have an "E. coli-like" smell, that may be recognizable to an olfactorily sophisticated microbiologist. Most species should be oxidase positive, nitrate reduction and catalase positive, alkaline phosphatase positive, and glucose fermenters. (Table 3)

Antimicrobial Susceptibilities

P. multocida usually is susceptible to penicillin, and it is the antimicrobial of choice for infections caused by this organism. Additional agents with proven efficacy include extended-spectrum cephalosporins, tetracycline and chloramphenicol. Although [beta]-lactamase-producing strains are isolated from animal sources, for the most part human wounds have not been found to harbor [beta]-lactamase-producing strains. In vitro susceptibility testing may not be necessary for isolates when multiple organisms are present since empiric therapy for mixed infections is usually effective against Pasteurella as well. Testing isolates from normally sterile body sites such as CSF, blood, joints, tissue or implanted devices may be appropriate. In 2006, The Clinical and Laboratory Standards Institute released the first international guidelines for testing a number of fastidious bacteria, including Pasteurella spp. In these guidelines, the following agents are suggested for testing if indicated: penicillin, a [beta]-lactam/ [beta]-lactamase inhibitor combination (e.g., ampicillin/sulbactam or piperacillin/tazobactam), ceftriaxone, moxifloxacin, levofloxacin, doxycycline, erythromycin or azithromycin and trimethoprim-sulfamethoxazole. Cation-adjusted Mueller-Hinton broth with 2 to 5% lysed horse blood is recommended for broth microdilution testing, while Mueller-Hinton agar with 5% sheep blood is recommended for diffusion disktesting. The recommended incubation is at 35[degrees]C for 18-24 hours in ambient air. The guidelines list a significant number of antimicrobial agents for both disk diffusion and dilution testing.


Bacterial meningitis caused by Pasteurella multocidia is a rare infection, especially among young infants and children. Meningitis in this age group are difficult to detect via physical exam because their lack of classic physical signs and symptoms, which include headache, photophobia (eye sensitivity to light), stiff neck, skin rashes, and seizures. However, meningitis should be suspected in the presence of fever and an unknown source of infection. An identification of Pasteurella spp. requires laboratory identification, combined with a careful history and physical examination. In this particular case, Pasteurella infection most likely came from close contact to household pets.

EDITOR'S NOTE: BEFORE reading the Case Follow-Up and Discussion below, study the Case Description on page 106 of this issue, and formulate your own answers to the questions posed.


(1.) Winn, W. (ed) et al, Koneman's Color Atlas and Textbook of Diagnostic Microbiology, 6th ed, 2006, Ch. 9, Miscellaneous Fastidious Gram-Negative Bacilli, pp. 458-467.

(2.) Murray, P (ed) et al, Manual of Clinical Microbiology, 9th ed, 2007, Ch. 40, Fastidious Gram-Negative Rods, pp. 622-623.

(3.) Kasper (ed) et al, Harrison's Principles of Internal Medicine, 16th ed, 2005. Ch. 360, Meningitis, Encephalitis, Brain Abscess and Empyema, Roos, K and Tyler, K, pp. 2471-2477

(4.) Donnio, P et al, Characterization of Pasteurella spp. strains isolated from human infections. J. Comp. Pathol. 130: 137-142.

(5.) Fell, H, Pasteurella multocida infection in West Suffolk, Journal of Infection, 9: 83-86.

(6.) Ahmed, M, Meningitis in a young infant, Clinical Pediatrics, 2004, 43: 495-496.

(7.) Tjen, C, Pasteurella meningo-encephalitis--a risk of household pets, Journal of Infection, 2004, 05:007.

(8.) Boocock, G and Bowley, J, Meningitis in Infancy caused by Pasteurella multocida, Journal of Infection, 1995, 31: 161-162.

(9.) Ellner, P and Correa-Londono, A, Pasteurella multocida as a cause of subdural empyema, Journal of Infection, 1981, 3:79-81.

(10.) Wrestling, K et al, Pasteurella multocida infection Following Cat Bites in Humans, Journal of Infection, 2000, 40:97-103.

(11.) Verhaegen, J., Actinobacillus (formerly Pasteurella) ureae meningitis and bacteraemia: report of a case and review of literature, Journal of Infection, 1988 17:249-253.

(12.) Castillo, M, Imagine of Meningitis, Seminars in Roentgenology, 2004 39(4): 458-464

(13.) Mortensen, J.E., O. Giger and G. Rodgers: In vitro Acticity of Oral Antimicrobial Agents Against Clinical Isolates of Pasteurella multocida. Diagn Microbiol Infect Dis 19989 Feb; 30(2):99-102.

(14.) Methods for Antimicrobial Dilution and Disk Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria; Approved Guidelines, M45-A. 2006. Clinical and Laboratory Standards Institute,Vol. 26, No. 19.

(15.) Naas, T. et. al, Molecular identification of TEM-1 beta-lactamase in a Pasteurella Multocida isolate of human origin. Eur. J. Clin. Microbio. Infect. Dis. 2L210-213.

George Chang, MD, Dept. of Pathology and Laboratory Medicine, University of Cincinnati College of Medicine; April Kilgore, MD, Division of Infectious Diseases, Cincinnati Children's Hospital; Ashley Bowman, MT, Depts. of Pathology and Laboratory Medicine, Cincinnati Children's Hospital; Joel E. Mortensen, PhD, Depts. of Pathology and Laboratory Medicine, Cincinnati Children's Hospital and University of Cincinnti College of Medicine
TABLE 1: Complete blood count and
chemistry analysis of serum

Analytes               Value                   Normal Range

                                    Complete Blood Count

WBC                     10.3 x [10.sup.9]/L          6.0-17.5x

WBC differential         15% neutrophile,            Segmented
                           3% bands, 70%       neturophils (54-62%)
                          lymphocyte and         Band forms (3-5%)
                           12% monocytes       Lymphocytes (24-44%)
                                                  Monocyte (3-6%)

                                        Renal Panel

Sodium                      136 mmol/L            133-142 mmol/L

Potassium                   4.2 mmol/L            3.5-5.6 mmol/L

Chloride                    102 mmol/L             96-10 mmol/L

Carbon dioxide               25 mmol/              17-29 mmol/L

BUN                          4.0 mg/dL              5-17 mg/dL

Creatinine                   0.3 mg/dL             0.2-0.4 mg/dL

                                      Inflammatory Index

C-reactive protein)          8.7 mg/dL               <1 mg/dL

ESR                          79 mm/hr               0-10 mm/hr

TABLE 2: Analysis of specimen from lumbar

Analyte             Value               Normal Range

White blood            944 [mm.sup.3]      0-4 [mm.sup.3]

differential          91 % neutrophils
                        8% monocytes
                       1% lymphocytes

glucose                   21 mg/dl          40-70 mg/dL

protein                  188 mg/dl           0-80 mg/dL

TABLE 3: Biochemical and Growth Characteristics of the more common
Pasteurella species

Species         Growth on       Beta
                MacConkey    hemolysis

P. multocida        -            -
P. gallinarum       -            -
P. bettyae         +/-           -
P. dagmatis         -            -
P. canis            -            -

Species           Indole       Urease     Ornithine

P. multocida        +            -            +
P. gallinarum       -            -            -
P. bettyae          +            -            -
P. dagmatis         +            +            -
P. canis           +/-           -            +

Species         Fermentation of

                 Maltose      Mannitol     Sorbitol     Sucrose

P. multocida        -            +            +           N/A
P. gallinarum       +            -           +/-           +
P. bettyae         +/-           -            -            -
P. dagmatis         +            -            -            +
P. canis            -            -            -            +

Adapted from reference 1
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Title Annotation:Article 357: 1 Clock Hour: CASES IN CLINICAL MICROBIOLOGY
Author:Chang, George; Kilgore, April; Bowman, Ashley; Mortensen, Joel E.
Publication:Journal of Continuing Education Topics & Issues
Article Type:Report
Geographic Code:1USA
Date:Aug 1, 2009
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