Less Commonly Encountered NonentericGram-Negative Bacilli

Pseudomonadaceae

(See also Chapter 154 and 155 .) Certain organisms in the family Pseudomonadaceae have been reclassified. An earlier classification scheme grouped the pseudomonads into 5 homology groups based on similarities in 16S rRNA gene sequence. At present, only the former members of rRNA homology group I are retained in the amended genus Pseudomonas. The most common Pseudomonas spp. isolated from human clinical specimens are the fluorescent pseudomonad group organisms, Pseudomonas aeruginosa, Pseudomonas putida, Pseudomonas fluorescens, Pseudomonas veronii, Pseudomonas monteillii, and Pseudomonas mosselii . They produce pyoveridin, a soluble yellow-green or yellow-brown pigment that fluoresces blue-green when exposed to ultraviolet light. The nonfluorescent group, which includes Pseudomonas stutzeri, Pseudomonas mendocina, Pseudomonas alcaligenes, Pseudomonas pseudoalcaligenes, Pseudomonas luteola, and Pseudomonas oryzihabitans, are isolated less frequently (see Chapter 154 ). Both P. luteola (formerly Chryseomonas luteola ) and P. oryzihabitans (formerly Flavimonas oryzihabitans ) are catalase-positive, oxidasenegative, motile, gram-negative bacilli that form yellow-pigmented, often wrinkled colonies on blood and MacConkey agar. The negative oxidase reaction of these organisms is unique among Pseudomonas spp. P. luteola can be differentiated from P. oryzihabitans on the basis of its ability to hydrolyze esculin and orthonitrophenyl-β- d -galactopyranoside. ^,

rRNA homology group II organisms now are designated Burkholderia spp., Ralstonia spp., Cupriavidus spp., Pandoraea spp., and related organisms in the family Burkholderiaceae ; rRNA homology group III contains Comamonas spp., Delftia spp., and Acidovorax spp. in the family Comamonadaceae ; rRNA homology group IV pseudomonads are now classified as Brevundimonas spp. in the family Caulobacteraceae ; and rRNA homology group V pseudomonads are now Stenotrophomonas spp.

Alcaligenaceae

The family Alcaligenaceae has undergone considerable reclassification. Both Achromobacter and Alcaligenes spp., along with the closely related genera, Kerstersia , Advenella , Paenalcaligenes , Oligella, and Bordetella currently are grouped together within the family . The clinically relevant Achromobacter and Alcaligenes spp. include Alcaligenes faecalis, Achromobacter piechaudii, Achromobacter denitrificans, and Achromobacter xylosoxidans. The first 3 organisms listed are asaccharolytic and are similar to Bordetella spp. and Oligella ureolytica biochemically. Achromobacter xylosoxidans is saccharolytic and is similar to several organisms biochemically, including Pannonibacter phragmitetus (formerly Achromobacter groups B, and F), Achchromabacter group F and Ochrobactrum anthropic (the former Achromobacter groups A, C, and D, now considered biovars of Ochrobactrum. anthropi) . Ochrobactrum spp. have been placed in the family Brucellaceae; they are oxidase positive, saccrolytic and motile. The genus Oligella contains 2 species of clinical significance, O. urethralis and O. ureolytica. O. urethralis (formerly Moraxella urethralis ) is nonmotile, shares several characteristics with Moraxella spp., and is thought to be a commensal of the genitourinary tract. O. ureolytica is motile and is similar to Bordetella bronchiseptica biochemically , including the property of rapid hydrolysis of urea.

Caulobacteraceae

Several Brevundimonas spp. have been named, but only Brevundimonas diminuta and Brevundimonas vesicularis are of human clinical significance . These species are placed in the family Caulobacteraceae . All these organisms are aerobic, oxidase positive, motile, gram-negative bacilli that can be isolated in the laboratory on blood or chocolate agar; most but not all strains also grow on MacConkey agar. Some isolates produce a yellow-orange or tan-brown pigment.

Comamonadaceae

Current members of the family Comamonadaceae include the following: Comamonas testosteroni and C. terrigena ; Delftia acidovorans (formerly Comamonas acidovorans ) and D. tsuruhatensis ; and 3 Acidovorax spp.— A. facilis, A. delafieldii, and A. temperans . These organisms grow on MacConkey agar and are oxidase- and catalase-positive.

Flavobacteriaceae

The family Flavobacteriaceae contains many genera, including Chryseobacterium, Elizabethkingia, Flavobacterium, Weeksella, Bergeyella, Empedobacter, and Myroides . Elizabethkingia meningoseptica (formerly Chryseobacterium meningosepticum and Flavobacterium meningosepticum ) and Chryseobacterium indologenes are the most common human isolates of this group. ^{, ,} E. meningoseptica produces large, smooth colonies on blood and chocolate agar within 24 hours, but most isolates do not grow on MacConkey agar. Gram stain of agar isolates reveals long, thin rods that can be filamentous. Recently two new species, E. anopheles and E. miricola , have been recognized and are considered emerging pathogens. Some studies suggest that E. anopheles and not E. meningospetica is actually the predominate human pathogen in the genus; however, accurate identification of these species is difficult. ^, The genus Chryseobacterium contains over 100 species and can be differentiated from other non-fermenters by its ability to produce indole in tryptophan broth. The human pathogens, in addition to C. indologenes, include C. gleum, C. anthropi, and C. hominis . These other Chryseobacterium spp. produce colonies of similar appearance; some produce yellow-pigmented colonies on blood agar, but pigment production may be negative or weak, except for the deep-yellow pigment produced by C. indologenes .

Flavobacterium breve has been reclassified as Empedobacter brevis. Organisms formerly classified as F. odoratum are now classified as 2 distinct species, Myroides odoratus and Myroides odoratimimus . Weeksella zoohelcum has been reclassified as Bergeyella zoohelcum, and Weeksella virosa remains as the single species in the genus. Unlike the other flavobacteria that are environmental organisms, W. virosa and B. zoohelcum are found on mucosal surfaces of humans and other mammals. ^, Colonies of W. virosa are mucoid, adherent to agar, and can develop a tan appearance, whereas colonies of B. zoohelcum can be sticky and tan in appearance. Useful tests to differentiate these organisms include the following: hydrolysis of gelatin, starch, and esculin; DNase and urease production; nitrate reduction; and susceptibility to penicillin and polymyxin B. ^,

Methylobacteriaceae and Acetobacteraceae

Methylobacterium spp. and Roseomonas spp. are members of the families Methylobacteriaceae and Acetobacteraceae, respectively. They are both oxidase positive and produce characteristic pink- or coral-pigmented colonies on a variety of media. The two genera are differentiated by the ability of Methalobacterium spp. to oxidize methanol. These are environmental organisms that also can be found in the healthcare environment. Colonies of Methylobacterium spp. generally appear dry, and Gram stain shows large, highly vacuolated, pleomorphic bacilli; growth on MacConkey agar at temperatures >40°C usually is negative. In contrast, colonies of Roseomonas spp. are mucoid in culture and show nonvacuolated, coccoid bacilli on Gram stain; these organisms usually grow on MacConkey agar at temperatures up to 42°C.

Other acetic acid bacteria belonging to the family Acetobacteraceae , besides Roseomonas , have been associated with human infection relatively recently. Relevant clinical genera include Asaia, Gluconobacter, and Granulibacter . These organisms oxidize alcohols or sugars, thus producing acetic acid. They are found in soil or associated with plants and have been used to convert wine to vinegar. Asaia spp. include a group of pink-pigmented gram-negative rods. Colonies are pale pink with scant to moderate growth on sheep blood agar, and they are oxidase negative. Relevant clinical species include Asaia bogorensis, Asaia lannensis, and Asaia siamensis . Gluconobacter are gram-negative, catalase-positive, oxidase-negative rods. Granulibacter are oxidase-negative, coccobacillary, gram-negative bacilli.

Rhizobiaceae

Rhizobium spp. and Agrobacterium spp. are members of the family Rhizobiaceae and are natural inhabitants of soil and well-known pathogens of plants. Rhizobium radiobacter (formerly Agrobacterium radiobacter and Agrobacterium tumefaciens ) is the only medically important species in the family. Colonies of the organism can become very mucoid and pink on MacConkey agar, resembling colonies of Klebsiella spp. It grows optimally at 25–28°C but also at 35°C but not at 41°C.