640 ACHROMOBACTERIUM 



ring growth. Produces large amount of gas in nitrate broth, reducing the nitrate to 



free nitrogen. Non-pathogenic. This organism is not to be confused with Ps. denitrificans 



{q.v. Chapter 21). 



ACHROMOBACTERIUM 



Definition. — Achromobacterium. 



Motile or non-motile, Gram-negative rods, usually small to medium in size, 

 forming no pigment on agar, and varying in their fermentative abihty. Optimum 

 temperature for growth about 25° C, but often good growth at 37° C. Saprophytic ; 

 commonly found in water, soil, and milk. 



Organisms of this group are widespread in nature, but have so far received little 

 systematic study. The public health bacteriologist meets them mainly in the 

 analysis of water, milk, food, and soil, where they attract attention by their fre- 

 quency on agar and gelatin plates incubated at 22° C. or sometimes at 37° C. 

 They are mainly responsible for the formation of slime on stored meat (Haines 

 1933). They are differentiated from the Cliromohacterium group mainly by their 

 failure to form pigment. Discussion of their classification would at present serve 

 no useful purpose, and all that we need do here is to give a brief account of their 

 more common characteristics. 



Morphologically they are Gram-negative, motile or non-motile rods, often of 

 the size of coliform bacilli, but varying considerably in thickness ; rather fat rods, 

 and fat cocco-bacilli are quite common. 



The optimum temperature for growth is about 20-25° C. At 37° C. growth 

 is generally stated to be slight or absent, but in our experience abundant growth 

 of these organisms is by no means infrequent in cultures made from certain foods. 

 Most strains grow at 0° C. (Coyne 1933) ; Haines (1933) found that the genera- 

 tion time in broth at this temperature was about 9 hours. The colonial appearances 

 vary. After 24 hours at 22° C. colonies on agar are about 0-5 mm. in diameter, 

 circular, smooth, convex, greyish white, and translucent with an entire edge ; after 

 5 days they are 3-5 mm. in diameter, raised or low convex, greyish, opaque, with 

 a smooth surface and entire edge, or sometimes with a beaten-copper surface and 

 an irregular edge. Mucoid colonies are not uncommon, and seem to be formed 

 most frequently by the short fat cocco-bacillary type. Colonies with central 

 crateriform depressions, draughtsman-like colonies, colonies with a roughish surface, 

 and colonies showing radial striation are sometimes met with. An aromatic odour 

 may be noticeable. 



In broth there is a uniform turbidity of varying degree with a powdery, granular, 

 or viscous deposit. On potato a layer of growth is formed, which is sometimes 

 mucoid or creamy, and which often takes on a cafe-au-lait appearance after a week 

 or two. 



Most, but not all, strains seem to grow in MacConkey's bile-salt medium. In 

 liquid MacConkey they give rise to turbidity, but not usually to acid production ; 

 on solid MacConkey they form small yellowish colonies, which after 5 days at 22° C. 

 may reach a diameter of 1-2 mm. Their growth is not inhibited by concentrations 

 of brilliant green and sodium tetrathionate, such as are used in the isolation of 

 Salmonella organisms ; and as they form non-lactose-fermenting colonies on 

 MacConkey's medium, they may cause trouble in the search for pathogenic organ- 



