Taxonomy,Role,and Significance of Microorganisms in Foods 21 more of the cited references should be consulted.Some of the phylogenetic features of these bacteria are presented in the Appendix. Acinetobacter(A.ci.ne'to.bac.ter:Gr.akinetos.unable to move).These Gram-negative rods 合2心eay Nesenaceae.nd some na wee6meOm are pl so,some f inetobacters are now in the genus Psychrobacte ey dip he l he nit many coccoid-shaned cells They are widely distributed in soil and water and may be found on man foods,especially refrigerated fresh products.The mol%G+Ccontent of DNA for the genus is 39-47 (See Chapter 4 for a further discussion relative to meats.)It has been proposed,based on DNA-rRNA hybridization data.that the genera Acinetobacter.Moraxella.and Psychrobacter be placed in a new family (Moraxellaceae).but this proposal has not been approved. m-negative rods formerly roduce c ous quantities of gas from the fermented sugars.They are normal inhabitants of the intestines of fish.and some are fish pathogens.The mol%G+C content of DNA is 57-65.(The species that possesses pathogenic properties is discussed in Chapter 31.) Alcaligenes (al.ca.li'genes:alkali producers).Although Gram negative,these organisms some Ihey are rods that sugg℃ all D urces.Thep ont nt of DNA 570.s ing that the Alteromonas(al.te.romo'nas;another monad).These are marine and coastal water inhabitants that are found in and on seafoods;all species require seawater salinity for growth.They are Gram-egative motile rods that are strict aerobe Thi nd ten was cre ated duringre rative curved or S-shaped rods thatare quite similar to the campyloba cme meme ae These oxidase-and catalase-positive organisms cause abortion and enteritis in some animals,and the latter in humans is associated with A.butzleri. Bacillus(ba·ci'lus These are Gram-positive spore-formin rods that are aerobes s in contrast t R R .d most strains of the latter are nonpatho ns.some cause foodbomne sastro enteritis (further discussed in Chapter 24).This genus has been delimited by the transfer of a number of its former species to eight new genera:Alicyclobacillus,Aneurinibacillus,Brevibacillus.Gracilibacillus,Paenibacillus. e食A&pgnc
Taxonomy, Role, and Significance of Microorganisms in Foods 21 more of the cited references should be consulted. Some of the phylogenetic features of these bacteria are presented in the Appendix. Acinetobacter (A • ci • ne’to • bac • ter; Gr. akinetos, unable to move). These Gram-negative rods show some affinity to the family Neisseriaceae, and some that were formerly achromobacters and moraxellae are placed here. Also, some former acinetobacters are now in the genus Psychrobacter. They differ from the latter and the moraxellae in being oxidase negative. They are strict aerobes that do not reduce nitrates. Although rod-shaped cells are formed in young cultures, old cultures contain many coccoid-shaped cells. They are widely distributed in soil and water and may be found on many foods, especially refrigerated fresh products. The mol% G + C content of DNA for the genus is 39–47. (See Chapter 4 for a further discussion relative to meats.) It has been proposed, based on DNA–rRNA hybridization data, that the genera Acinetobacter, Moraxella, and Psychrobacter be placed in a new family (Moraxellaceae), but this proposal has not been approved. Aeromonas(ae • ro • mo’nas; gas producing). These are typically aquatic Gram-negative rods formerly in the family Vibrionaceae but now in the family Aeromonadaceae.32 As the generic name suggests, they produce copious quantities of gas from the fermented sugars. They are normal inhabitants of the intestines of fish, and some are fish pathogens. The mol% G + C content of DNA is 57–65. (The species that possesses pathogenic properties is discussed in Chapter 31.) Alcaligenes (al • ca • li’ge • nes; alkali producers). Although Gram negative, these organisms sometimes stain Gram positive. They are rods that do not, as the generic name suggests, ferment sugars but instead produce alkaline reactions, especially in litmus milk. Nonpigmented, they are widely distributed in nature in decomposing matter of all types. Raw milk, poultry products, and fecal matter are common sources. The mol% G + C content of DNA is 58–70, suggesting that the genus is heterogeneous. Alteromonas(al • te • ro • mo’nas; another monad). These are marine and coastal water inhabitants that are found in and on seafoods; all species require seawater salinity for growth. They are Gram-negative motile rods that are strict aerobe.17 Arcobacter (Ar’co • bac • ter; L. arcus, bow). This genus was created during revision of the genera Campylobacter, Helicobacter, and Wolinella, 45 and the three species were once classified as Campylobacter. They are Gram-negative curved or S-shaped rods that are quite similar to the campylobacters except they can grow at 15◦C and are aerotolerant. They are found in poultry, raw milk, shellfish, and water; and in cattle and swine products.51,52 These oxidase- and catalase-positive organisms cause abortion and enteritis in some animals, and the latter in humans is associated with A. butzleri. Bacillus (ba • cil’lus). These are Gram-positive spore-forming rods that are aerobes in contrast to the clostridia, which are anaerobes. Although most are mesophiles, psychrotrophs and thermophiles exist. The genus contains only two pathogens: B. anthracis (cause of anthrax) and B. cereus. Although most strains of the latter are nonpathogens, some cause foodborne gastroenteritis (further discussed in Chapter 24). This genus has been delimited by the transfer of a number of its former species to eight new genera: Alicyclobacillus, Aneurinibacillus, Brevibacillus, Gracilibacillus, Paenibacillus, Virgibacillus, and Salibacillus. 5 Also, the former group 5 Bacillus species are now in the genus Geobacillus, and the former B. stearothermophilus is now G. stearothermophilus.36
22 MODERN FOOD MICROBIOLOGY Brevibacillus (B eeoead re common on plants. Brochothrix (bro chothr'ix:Gr.brochos,loop:thrix,thread).These Gram-positive nonspore- forming rods are closely related to the genera Lactobacillus and Listeria,and some of the common features are discussed in Chapter 25.Although they are not true coryneforms,they bear resemblance to this group.Typically,exponential-phase cells are rods,and older cel are Is,a le ture typica or coryne The cstatus nas been rea d by rRNA though and on fresh and eats that are stored in gas-impermeable packages at refriger tor temne eratures.In contrast to B.thermo snhacta B.campestris is rhamnose and hippurate positive.4 The mol%G+Ccontent of DNA is 36.They do not grow at 37C. e(Burkholder)Gram-negative rods that oceur on pepe 26ad cially certain owers） cause vegeta cepacid.a stud ficant pathogens in ystic fibrosis patients he genus P are sign Campylobacter(cam·pw'·lo·bac ter:Gr.campylo.curved).Although most often pronounced "camp'lobacter."the technically correct pronunciation should be noted.These Gram-negative,spi- rally curved rods were formerly classified as vibrios.They are microaerophilic to anaerobic.The genus has been restructured since 1984.The once C.nitrofigilis and C. cryaerophila have been transferred to acrer.the on are n in the genus acte nd w The m 37 28 Carnobacterium (car.no.bac.terium;L.carnis,of flesh-meat bacteria).This genus of Gram- They pyee They are heterofermentative,and most grow at C and none at 45C Gas is produced from glucos by some spe and the mol%G+C or the ti nus 33 m the lactoba ed pro e foun Citrobacter (cit.ro.bac'ter)Theseenteric bacteria are slow lactose-fermenting Gram-negative rods that typically produce yellow colonies on plate count agar.All members can use citrate as the sole m The m C流DAw2aem Clostridium(clos·tni'di·umGr.clo er,aspindle).Th spore-forming rods are widel some of which c use dise botulism).Mesotrophic. mportance in the thermal canning of foods is discussed in Chapter 17.A reorganization of the genus created the following five new genera:Caloramater,Filifactor,Moorella,Oxobacter,and Oxalophagus.The
22 Modern Food Microbiology Brevibacillus (Bre • vi • ba • cil’lus). Previously classified as Bacillus spp. as noted above, these organisms occur in soil and water, and are common on plants, and in air, and dust. At least nine species are recognized. Brochothrix (bro • cho • thr’ix; Gr. brochos, loop; thrix, thread). These Gram-positive nonsporeforming rods are closely related to the genera Lactobacillus and Listeria, 40 and some of the common features are discussed in Chapter 25. Although they are not true coryneforms, they bear resemblance to this group. Typically, exponential-phase cells are rods, and older cells are coccoids, a feature typical of coryneforms. Their separate taxonomic status has been reaffirmed by rRNA data, although only two species are recognized: B. thermosphacta and B. campestris. They share some features with the genus Microbacterium. They are common on processed meats and on fresh and processed meats that are stored in gas-impermeable packages at refrigerator temperatures. In contrast to B. thermosphacta, B. campestris is rhamnose and hippurate positive.44 The mol% G + C content of DNA is 36. They do not grow at 37◦C. Burkholderia (Burkholder • ia). Gram-negative rods that occur on plants (especially certain flowers), in raw milk, and cause vegetable spoilage. In a study of raw cow’s milk in Northern Ireland, 14 out of 26 (54%) samples contained B. cepacia.34 They are significant pathogens in cystic fibrosis patients. They were formerly classified in the genus Pseudomonas. Campylobacter (cam • py’ • lo • bac • ter; Gr. campylo, curved). Although most often pronounced “camp’lo • bac • ter,” the technically correct pronunciation should be noted. These Gram-negative, spirally curved rods were formerly classified as vibrios. They are microaerophilic to anaerobic. The genus has been restructured since 1984. The once C. nitrofigilis and C. cryaerophila have been transferred to the new genus Arcobacter; the once C. cinnaedi and C. fenneliae are now in the genus Helicobacter; and the once Wolinella carva and W. recta are now C. curvus and C. rectus.45 The mol% G + C content of DNA is 30–35. For more information, see reference 32 and Chapter 28. Carnobacterium (car • no • bac • terium; L. carnis, of flesh-meat bacteria). This genus of Grampositive, catalase-negative rods was formed to accommodate some organisms previously classified as lactobacilli. They are phylogenetically closer to the enterococci and vagococci than the lactobacilli.6,12 They are heterofermentative, and most grow at 0◦C and none at 45◦C. Gas is produced from glucose by some species, and the mol% G + C for the genus is 33.0–37.2. They differ from the lactobacilli in being unable to grow on acetate medium and in their synthesis of oleic acid. They are found on vacuum-packaged meats and related products, as well as on fish and poultry meats.11,23,48 Citrobacter (cit • ro • bac’ter). These enteric bacteria are slow lactose-fermenting, Gram-negative rods that typically produce yellow colonies on plate count agar. All members can use citrate as the sole carbon source. C. freundii is the most prevalent species in foods, and it and the other species are not uncommon on vegetables and fresh meats. The mol% G + C content of DNA is 50–52. Clostridium (clos • tri’di • um; Gr. closter, a spindle). These anaerobic spore-forming rods are widely distributed in nature, as are their aerobic counterparts, the bacilli. The genus contains many species, some of which cause disease in humans (see Chapter 24 for C. perfringens food poisoning and botulism). Mesotrophic, psychrotrophic, and thermophilic species/strains exist; their importance in the thermal canning of foods is discussed in Chapter 17. A reorganization of the genus created the following five new genera: Caloramater, Filifactor, Moorella, Oxobacter, and Oxalophagus. 8 The
Taxonomy,Role,and Significance of Microorganisms in Foods 23 clostridial species of known importance in foods remain in the genus at this time.The five new genera appear to be unimportant in foods. Corynebacterium(coryne ed inth of the true coryne cte Mo nhe The us has bee with the plant path ens to the genus Clavibacter and others to the ger content of DNA is 51-63. obacter ( 'ter).These enter negative bacteria are typical estinal tract The dis ed in c 20 E asglom ns has o transferred to the genus Pantoed.E.sakazakii is discussed in Chapter 31. occus(em· coc us).This genus some of the Lancefiel group D occi.I o more mnan Io s in pairs he genus Str D 20 nd its phyl napte genetic cid hac an be s een tran E.ca ovord and E.cnrysa n the genus Pectobacterium as P.carovovorum and ew(see Chapter6】 Escherichia (escher.i'chia).This is clearly the most widely studied genus of all bacteria.Those strains that cause foodborne gastroenteritis are discussed in Chapter 27,and E.coli as an indicator of food safety is discussed in Chapter 20. um (fla.vo bac.te'ri.um).Th duction riedby their p nd h the vith and others are psvchrotrophs.where they participate in the spoilage of refrigerated meats and vegeta- bles.Some of the former flavobacterial species have been placed in the following five new genera: Empedobacter.Chryseobacterium.Myroides.Sphingomonas,and Sphingobacterium. Hafnia (haf'ni·a.Th nt in the and ve H al iis th orithine positive.and it has a mol%G+C content of DNA of 48-49. Kocuria(Ko· cu'ria after M.Kocur).A new genus split off from the genus Micrococcus.2 The the mo ,h.Vo kristinae)are oxidase negative and catalase positive,and +C content of DNA is 66-75 Lactobacillus(lactobacil'lus).Taxonomic techniques that came into wide use during the 1980s have been applied to this genus,resulting in some of those in the ninth edition of Bergey's Manual being transferred to other genera.Based on 16S rRNA sequence data,three phylogenetically distinct sters are revea th one cluster en ompass ng Weissella.In all proba ility,th s genus v undergo further reclassification.They are Gram-positive,catalase-negative rods that
Taxonomy, Role, and Significance of Microorganisms in Foods 23 clostridial species of known importance in foods remain in the genus at this time. The five new genera appear to be unimportant in foods. Corynebacterium (co • ry • ne • bac • ter’ • i • um; Gr. coryne, club). This is one of the true coryneform genera of Gram-positive, rod-shaped bacteria that are sometimes involved in the spoilage of vegetable and meat products. Most are mesotrophs, although psychrotrophs are known, and one, C. diphtheriae, causes diphtheria in humans. The genus has been reduced in species with the transfer of some of the plant pathogens to the genus Clavibacter and others to the genus Curtobacterium. The mol% G + C content of DNA is 51–63. Enterobacter (en • te • ro • bac’ter). These enteric Gram-negative bacteria are typical of other Enterobacteriaceae relative to growth requirements, although they are not generally adapted to the gastrointestinal tract. They are further characterized and discussed in Chapter 20. E. agglomerans has been transferred to the genus Pantoea. E. sakazakii is discussed in Chapter 31. Enterococcus (en • te • ro • coc’cus). This genus was erected to accommodate some of the Lancefield serologic group D cocci. It has since been expanded to more than 16 species of Gram-positive ovoid cells that occur singly, in pairs, or in short chains. They were once in the genus Streptococcus. Some species do not react with group D antisera. The genus is characterized more thoroughly in Chapter 20, and its phylogenetic relationship to other lactic acid bacteria can be seen in Figure 25–1. Erwinia (er • wi’ni • a). These Gram-negative enteric rods are especially associated with plants. At least three species have been transferred to the genusPantoea, 33 and the formerE. carotovora andE. chrysanthemi are now in the genus Pectobacterium as P. carovovorum and P. chrysanthemi (see Chapter 6). Escherichia (esch • er • i’chi • a). This is clearly the most widely studied genus of all bacteria. Those strains that cause foodborne gastroenteritis are discussed in Chapter 27, and E. coli as an indicator of food safety is discussed in Chapter 20. Flavobacterium (fla • vo • bac • te’ri • um). These Gram-negative rods are characterized by their production of yellow to red pigments on agar and by their association with plants. Some are mesotrophs, and others are psychrotrophs, where they participate in the spoilage of refrigerated meats and vegetables. Some of the former flavobacterial species have been placed in the following five new genera: Empedobacter, Chryseobacterium, Myroides, Sphingomonas, and Sphingobacterium. Hafnia (haf’ni • a). These Gram-negative enteric rods are important in the spoilage of refrigerated meat and vegetable products; H. alvei is the only species at this time. It is motile and lysine and ornithine positive, and it has a mol% G + C content of DNA of 48–49. Kocuria (Ko • cu’ri • a, after M. Kocur). A new genus split off from the genus Micrococcus.42 The three species (K. rosea, K. varians, and K. kristinae) are oxidase negative and catalase positive, and the mol% G + C content of DNA is 66–75. Lactobacillus (lac • to • ba • cil’lus). Taxonomic techniques that came into wide use during the 1980s have been applied to this genus, resulting in some of those in the ninth edition of Bergey’s Manual being transferred to other genera. Based on 16S rRNA sequence data, three phylogenetically distinct clusters are revealed,10 with one cluster encompassing Weissella. In all probability, this genus will undergo further reclassification. They are Gram-positive, catalase-negative rods that often occur in
24 MODERN FOOD MICROBIOLOGY long chains.Alt P n.On Many fermented products are produced,and these are discussed in Chapter 7.Those that are common on refrigerator-stored,vacuum-packaged meats are discussed in Chapters 5 and 14. (actoco'cus).The nonmotile Lancefield serologic group N cocci once classifi the genus s re ococcus have een eleva a to genene statu y are mm-pos 10C but not at 45C d most strains eact with g oup N an an end-product of fermentation Leuconostoc (leuconos'toc:colorless nostoc).Along with the lactobacilli.this is another of the genera of lactic acid bacteria.They are Gram-positive,catalase-negative cocci that are heterofermen- la below).The former L.oeno. en tran to a new genus, hew genus Weiss and the former These cocci are typically found on with Listeria (lis.te'ri.a).This genus of six species of Gram-positive.nonsporing rods is closely related to Brochothrix.The six species show 80%similarity by numerical taxonomic studies;they have identical cell walls,fatty acid,and cytochrome composition.They are more fully described and discussed in Chapter 25 Miccocim coc'cus) These Gram- and cat tive cocci are inhabitan by the creation of the fol o five ne era:Der us,Kocuria,Kyte Nest and Stomatococcus.At the present time,M.luteus and M.lvlae are the only two micrococcal species Moraxella(moraxel'la).These short Gram-negative rods are sometimesclassified as Acinetobacter They difter from the sensitive to penicillin and oxidase positive and havingamo 40. e genus s some that were once placed in Paenibacillus (pae.ba.cil'lus:almost a bacillus).This newly established s formerly in the genera Bacillus and Clostridium.and it includes the following species:P alvei. P amylolyticus,P.azotofixans,P circulans,P.durum,P.larvae,P macerans,P.macquariensis P.pubuli,P pulvifaciens,and P.validus.28 Recently,two new species were added (P.lautus and P peoriae).The paenibacilli are notable for their degradation of a number of macromolecules,their actena t some【 s.A newly-named species was ed from raw an Pandoro outa of cystic fibrosis patients.7 these foods,one species,P norimbergenesis,has been isolated from powdered milk.3
24 Modern Food Microbiology long chains. Although those in foods are typically microaerophilic, many true anaerobic strains exist, especially in the colon and the rumen. They typically occur on most, if not all, vegetables, along with some of the other lactic acid bacteria. Their occurrence in dairy products is common. One species, L. suebicus, was recovered from apple and pear mashes; it grows at pH 2.8 and in 12–16% ethanol.28 Many fermented products are produced, and these are discussed in Chapter 7. Those that are common on refrigerator-stored, vacuum-packaged meats are discussed in Chapters 5 and 14. Lactococcus (lac • to • coc’cus). The nonmotile Lancefield serologic group N cocci once classified in the genus Streptococcus have been elevated to generic status. They are Gram-positive, nonmotile, and catalase-negative, spherical, or ovoid cells, that occur singly, in pairs, or as chains. They grow at 10◦C but not at 45◦C, and most strains react with group N antisera. l-Lactic acid is the predominant end-product of fermentation. Leuconostoc (leu • co • nos’toc; colorless nostoc). Along with the lactobacilli, this is another of the genera of lactic acid bacteria. They are Gram-positive, catalase-negative cocci that are heterofermentative. The genus has been reduced in number of species (see Weissella below). The former L. oenos has been transferred to a new genus, Oenococcus as O. oeni, 16 and the former L. paramesenteroides has been transferred to the new genus Weissella. These cocci are typically found in association with the lactobacilli. Listeria (lis • te’ri • a). This genus of six species of Gram-positive, nonsporing rods is closely related to Brochothrix. The six species show 80% similarity by numerical taxonomic studies; they have identical cell walls, fatty acid, and cytochrome composition. They are more fully described and discussed in Chapter 25. Micrococcus (mi • cro • coc’cus). These Gram-positive and catalase-positive cocci are inhabitants of mammalian skin and can grow in the presence of high levels of NaCl. This genus has been reduced by the creation of the following five new genera: Dermacoccus, Kocuria, Kytococcus, Nesterenkonia, and Stomatococcus. At the present time, M. luteus and M. lylae are the only two micrococcal species. Moraxella (mo • rax • el’la). These short Gram-negative rods are sometimes classified asAcinetobacter. They differ from the latter in being sensitive to penicillin and oxidase positive and having a mol% G + C content of DNA of 40–46. The genus Psychrobacter includes some that were once placed in this genus. Their metabolism is oxidative, and they do not form acid from glucose. Paenibacillus(pae • ba • cil’lus; almost a bacillus). This newly established genus comprises organisms formerly in the genera Bacillus and Clostridium, and it includes the following species: P. alvei, P. amylolyticus, P. azotofixans, P. circulans, P. durum, P. larvae, P. macerans, P. macquariensis, P. pubuli, P. pulvifaciens, and P. validus. 2,8 Recently, two new species were added (P. lautus and P. peoriae22). The paenibacilli are notable for their degradation of a number of macromolecules, their production of antibacterial and antifungal agents, and the capacity of some to fix N2 in association with plants. A newly-named species was isolated from raw and UHT-treated milk.38 Pandoraea (Pan • do • rae’a). Although first isolated from sputa of cystic fibrosis patients,7 these organisms are related to some of the pseudomonads. Although not demonstrated to be common in foods, one species, P. norimbergenesis, has been isolated from powdered milk.35
Taxonomy,Role,and Significance of Microorganisms in Foods 25 Pantoea (pan.toe'a).This genus consists of Gram-negative.noncapsulated.nonsporing straigh rods,most of which are motile by peritrichous flagella.They are widely distributed and are found on plants and in seeds.in soil,water,and human specimens.Some are plant pathogens.The four recognized species were once classified as enterobacters or erwinias.P agglomerans includes the fommer Enterobacter agglomerans.Erwinia herbicola,and E.milleriae:P.ananas includes the forme mgesiom497o06mo统ndRhperainomigna Pediococcus (pe.di.o.coc'cus:coccus growing in one plane).These homofermentative cocci are lactic acid bacteria that exist in pairs and tetrads resulting from cell division in two planes.P acidilactici,a common starter species,caused septicemia ina53-year-old male.Their mol%G+C content of DNA is 34-44:they are further discussed in Chapter 7.The once P.halophilus is now in the genus Tetragenococcus as T.halophilus.It can grow in 18%NaCl. ram-nega s are aerobes s that often dis omorphism agar plate The nical of enteric ha animals.They may be isolated from a variety of vegetable and meat products,especially those that undergo spoilage at temperatures in the mesophilic range. Pseudomonas(pseudo'monas;false monad).These are typical soil and water bacteria and they among fres a eg eats.poult ry.an pro ucts. the by the Burkholderia.Cor sia H stonia sphingomonas telluria and wautersia p fuc ens and p ae genus(see reference 24). Psychrobacter(psychrobacter).This genus was created primarily to accommodate some of the nmotile Gram-negative e genera Ac and M l1 that ofter cur i P 6.5%Naccata and n ().Theyare sensitive to whereas the acinetobacter do not.They are distinguished from the acinetobacters by being oxidase positive and aminovalerate users and from nonmotile pseudomonads by their inability to utilize glvcerol or fructose.Because they closely resemble the moraxellae,they have been placed in the family Neisseriaceae.The genus Salmonella (sal.mon.el'la)All membe of this of Gram sidered to he human pathoe with those that affect humans placed in the species Salmonella enterica.The serotypes (serovars)of more than 2,400 are listed as follows:Salmonella enterica serotype Newport,or Salmonella Newport (note that the serotype is not italicized).(See Chapter 26 for a more detailed explanation.)The mol G+C content of DNA is 50-53
Taxonomy, Role, and Significance of Microorganisms in Foods 25 Pantoea (pan • toe’a). This genus consists of Gram-negative, noncapsulated, nonsporing straight rods, most of which are motile by peritrichous flagella. They are widely distributed and are found on plants and in seeds, in soil, water, and human specimens. Some are plant pathogens. The four recognized species were once classified as enterobacters or erwinias. P. agglomerans includes the former Enterobacter agglomerans, Erwinia herbicola, and E. milletiae; P. ananas includes the former Erwinia ananas and E. uredovora; P. stewartii was once E. stewartii; and P. dispersa is an original species.18 The G + C content of DNA ranges from 49.7 to 60.6 mol%.33 Pediococcus (pe • di • o • coc’cus; coccus growing in one plane). These homofermentative cocci are lactic acid bacteria that exist in pairs and tetrads resulting from cell division in two planes. P. acidilactici, a common starter species, caused septicemia in a 53-year-old male.19 Their mol% G + C content of DNA is 34–44; they are further discussed in Chapter 7. The once P. halophilus is now in the genus Tetragenococcus as T. halophilus. It can grow in 18% NaCl. Proteus (pro’te • us). These enteric Gram-negative rods are aerobes that often display pleomorphism, hence the generic name. All are motile and typically produce swarming growth on the surface of moist agar plates. They are typical of enteric bacteria in being present in the intestinal tract of humans and animals. They may be isolated from a variety of vegetable and meat products, especially those that undergo spoilage at temperatures in the mesophilic range. Pseudomonas (pseu • do’mo • nas; false monad). These are typical soil and water bacteria and they are widely distributed among fresh foods, especially vegetables, meats, poultry, and seafood products. Although once the largest genus of foodborne bacteria, the genus has been delimited by the transfer of many former species to at least 13 new genera: Acidovorax, Aminobacter, Brevundimonas, Burkholderia, Comamonas, Delftia, Devosia, Herbaspirillium, Hydrogenophaga, Marinobacter, Ralstonia, Sphingomonas, Telluria, and Wautersia. P. fluorescens and P. aeruginosa remain in the original genus (see reference 24). Psychrobacter (psy • chro’ • bac • ter). This genus was created primarily to accommodate some of the nonmotile Gram-negative rods that were once classified in the genera Acinetobacter and Moraxella. They are plump coccobacilli that often occur in pairs. Also, they are aerobic, nonmotile, and catalase and oxidase positive, and generally do not ferment glucose. Growth occurs in 6.5% NaCl and at 1◦C, but generally not at 35◦C or 37◦C. They hydrolyze Tween 80, and most are egg-yolk positive (lecithinase). They are sensitive to penicillin and utilize γ -aminovalerate, whereas the acinetobacters do not. They are distinguished from the acinetobacters by being oxidase positive and aminovalerate users and from nonmotile pseudomonads by their inability to utilize glycerol or fructose. Because they closely resemble the moraxellae, they have been placed in the family Neisseriaceae. The genus contains some of the former achromobacters and moraxellae, as noted. They are common on meats, poultry, and fish, and in water.26,39 Salmonella (sal • mon • el’la). All members of this genus of Gram-negative enteric bacteria are considered to be human pathogens. It should be noted that the salmonellae have been placed in two species with those that affect humans placed in the species Salmonella enterica. The serotypes (serovars) of more than 2,400 are listed as follows: Salmonella enterica serotype Newport, or Salmonella Newport (note that the serotype is not italicized). (See Chapter 26 for a more detailed explanation.) The mol% G + C content of DNA is 50–53