1887

Diagnosis of bacterial, fungal and mycobacterial diseases

image of Diagnosis of bacterial, fungal and mycobacterial diseases
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Abstract

Microorganisms interact continuously with animal hosts. Occasionally this leads to infection and disease in the host. Detection and identification of an infecting microorganism depends on good sampling technique, competent laboratory methodology and an understanding of the pathogenicity of specific microorganisms. This chapter looks at collection and storage of samples for culture, microscopy and staining, bacterial and fungal identification, significance of bacterial isolates, PCR testing, antigen detection and serology, antimicrobial susceptibility testing, in-practice bacterial culture and antimicrobial susceptibility testing and multidrug-resistant bacterial pathogens. The chapter also includes case examples.

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Figures

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27.2 Anaerobic transport devices. (Courtesy and © Becton, Dickinson and Company, reprinted with permission)
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27.3 Prostatic wash from a dog containing , a macrophage and neutrophils, two with phagocytosed bacterial bacilli. Phagocytosis of bacteria by neutrophils supports bacterial infection. (Modified Wright–Giemsa stain; original magnification X1000)
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27.4 Lymph node aspirate from a dog with lymphoma and bacterial lymphadenitis. The image shows neutrophils, one lower right with phagocytosed bacterial cocci, degenerate cells, a small lymphocyte and a neoplastic lymphoid cell. (Modified Wright–Giemsa stain; original magnification X1000)
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27.5 , a common human pathogen and occasional pathogen in cats. Degenerate erythrocytes and characteristic paired bacterial cocci with non-staining capsules are visible. (Modified Wright–Giemsa stain; original magnification X1000)
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27.6 Urine sediment from a dog with infection; bacterial bacilli and neutrophils with one crenated erythrocyte are visible. (Papanicolaou stain; original magnification X1000)
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27.7 Discharge from a cat spay wound. Probable species; non-staining bacterial bacilli and a macrophage left of centre with intracellular bacteria are visible. (Modified Wright–Giemsa stain; original magnification X1000) (Courtesy of M Dunlop)
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27.8 Discharge from a cat spay wound (same case as Figure 27.7 ). Probable species; acid-fast bacilli stain pink/red against a blue background and the macrophage left of centre contains intracellular bacteria. (Ziehl–Neelsen stain; original magnification X1000) (Courtesy of M Dunlop)
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27.9 Smear of ear discharge from a dog. ; characteristic broad-based apical budding yeasts and anucleate superficial squamous epithelial cells are seen. (Modified Wright–Giemsa stain; original magnification X1000)
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27.10 Nasal discharge from a dog with infection. Fungal elements (hyphae) are seen, with numerous free conidia (spores). (Modified Wright–Giemsa stain; original magnification X1000) (Courtesy of M Serra)
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27.11 Submandibular lymph node aspirate from a cat. yeasts are visible, with thick non-staining polysaccharide capsules. (Modified Wright–Giemsa stain; original magnification X1000) (Courtesy of M Serra)
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27.12 Fungal hyphae in a splenic capsule from a dog. Systemic mycosis in this 5-year old Whippet occurred secondary to azathioprine therapy. (Haematoxylin and eosin stain; original magnification X400) (Courtesy of J Hargreaves)
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27.13 Meticillin-resistant growth on a selective chromogenic agar. Blue colonies are presumed positive for meticillin-resistant spp.
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27.14 Identification of Gram-positive coccoid bacteria. Lancefield grouping is a method of grouping catalase-negative, coagulase-negative bacteria based on the carbohydrate composition of bacterial antigens found on the cell walls. was believed to be a member of the genus at the time the Lancefield grouping was devised. MIC = minimum inhibitory concentration.
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27.15 The tube coagulase test: rabbit plasma is incubated with a bacterial suspension. Fibrinogen is converted to fibrin by coagulase enzyme to give a clot in the right-hand tube, a positive test. The left-hand tube is clear and therefore negative.
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27.16 API test strip for the biochemical identification of spp.
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27.17 Direct smear of a cystocentesis urine sample from a dog. Transitional epithelial cells, neutrophils and filamentous bacterial bacilli are visible. (Modified Wright–Giemsa stain; original magnification X1000)
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27.18 Cytospin smear (same case as in Figure 27.17 ) of the plain cystocentesis urine sample viewed 24 hours later. Degenerate nucleated cells with bacterial cocci are present. was isolated. This is likely to represent overgrowth in the plain urine sample by a skin surface contaminant introduced during cystocentesis. (Modified Wright–Giemsa stain; original magnification X1000)
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27.22 Pus expressed from a deep pyoderma lesion in a dog with concurrent demodicosis. (Courtesy of R Wilkinson)
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27.23 -like cutaneous lesion in a cat. (Courtesy of D Gunn-Moore)
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27.25 Manual nucleic acid extraction for PCR testing.
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27.26 Agar gel disc diffusion: α-haemolytic species from canine urine grown on Mueller–Hinton agar with 5% sheep blood. Susceptible to cefavecin (CVN) and erythromycin (E). Intermediate susceptibility to marbofloxacin (MAR).
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27.29 E-test MIC testing. (a) The cefalexin (CE) strip does not inhibit bacterial growth (bacteria have grown around the whole length of the strip); the MIC is given as >256 μg/ml. (b) The amoxicillin/clavulanic acid strip (XL) shows a narrow elliptical zone of inhibition that intersects the MIC scale where the concentration of the antimicrobial inhibits bacterial growth (arrowed); the MIC is given as 128 μg/ml.
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27.30 Mediolateral oblique view of the lower hindlimb of the cat. (Courtesy of K Alexander)
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27.31 Impression smear of a sample obtained from a discharging sinus. (Diff Quik® stain; original magnification X1000) (Courtesy of K Alexander)

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