The type species (ATCC 13637) and one other strain (ATCC 17444) of Pseudomonas maltophilia have been examined for their ability to use the disaccharide lactose for growth. These organisms lack the usual high levels of 0-galactosidase found in most lactose-positive bacteria, although both strains were found to have high levels of the enzyme lactose dehydrogenase. If this latter enzyme is responsible for all lactose splitting, P. rnaltophilia is unlike other pseudomonads, which have lactose dehydrogenase but which apparently cannot use lactose as a source of carbon and energy for growth.Aithough Pseudornonas rnaltophilia has been widely isolated from clinical specimens, water, and raw milk (4), and is probably of widespread occurrence, little is known about either its ecology or its physiology. It does, however, while presenting a remarkably uniform biotype, appear to be nutritionally less versatile than the other aerobic pseudomonads (16). The multitrichous P. maltophilia does not accumulate poly-0-hydroxybutyrate as a cellular reserve material and it cannot attack exogenous polymers. Methionine is universally required as a growth factor (5, 16), and all strains are oxidase negative, hydrolyze gelatin, and are strongly lipolytic. Nitrate cannot be used as a principal nitrogen source, and organic compounds are only utilized to a limited extent as compared to the other aerobic pseudomonads. Notable, however, is its ability to use the disaccharides lactose, maltose, and cellobiose. Cell growth with lactose as substrate is slow and low cell yields are obtained Previously, the growth of an organism with lactose as the sole carbon source was considered to indicate the presence of P-galactosidase (EC 3.2.1.23 @-D-galactoside galactohydrolase) (8). However, the breakdown of lactose to more utilizable intermediates can be mediated by enzymes other than P-galactohydrolase.Recent work has established the presence of an enzyme, P-D-phosphogalactoside galactohydrolase, in some microbes which hydrolyzes lactose-phosphate after phosphorylation by a phosphoenolpyruvate (PEP) transferase type of system (6-9, 11, 14, 15). Other enzymes found in pseudomonads (such as P. graueolans) catalyze the formation of lactobionic acid via lactobionic 6-lactone after direct oxidation of lactose by two enzymes. lactose dehydrogenase and lactobionic 6-lactone lactonase (1,12,13). No carbon atoms in the resulting compound, lactobionic acid, are used for cell synthesis. However, in Streptococcus lactis UN, an organism which has the same enzymes (17), lactobionic acid is further hydrolyzed to galactose and gluconate.The present study was initiated in an attempt to determine the reasons why the routine o-nitrophenyl-@-D-galactopyranoside (ONPG) disk test (Difco) carried out on both P. multophilia strains showed the presence of small amounts of 0-galactosidase in P . maltophilia strain 67 (a strain barely able to use lactose in a minimal medium assimilation test), while P. maltophiliu strain 68 showed only trace activity in the (ONPG) disk test but u...