Enteric Gram negative Rods

1
Enteric Gram negative Rods

• Enterobacteriaceae

2
Introduction

• The family Enterobacteriaceae (enterics) contains
  Gram (-) facultative rods that are common
  intestinal (colon) microbes, most of which are
  either commensal or even mutualistic in the
  colon.
• These are the most frequently isolated bacteria
  in the clinical lab coming from fecal samples
  infections of fecal origin. They also colonize
  mucous membranes outside the gut, especially in
  hospitalized patients
• In extraintestinal sites they are frequently
  associated with UTI, lower respiratory tract and
  wound infections. In fact, there is hardly an
  anatomic site they do not infect in some cases.
• A few genera are not normal colon microbes - they
  are primary intestinal pathogens (a.k.a.
  enteropathogenic)
• The enteropathogenic enterics cause GI conditions
  such as diarrhea, enteritis, dysentery, and
  enteric fevers (e.g. typhoid and paratyphoid)

3
Introduction

• Most genera are easily grown, and most look very
  similar making presumptive ID based upon
  morphology relatively difficult
• cellular small, short Gram (-) rods in singles
  and pairs
• colonial off-white, translucent, slightly convex
  colonies
• Proteus and Klebsiella are exceptions. Proteus
  (waves on the ocean) swarms on an agar
  surface, and Klebsiella forms very mucoid
  colonies.
• Another exception is Yersinia entercolitica
  because it grows better at room temp than at body
  temp. Y. entercolitica colonies are 1 mm or less
  in diameter after 24h incubation on blood or
  MacConkey agar at 35oC whereas typical enterics
  produce colonies 2-4 mm in diameter in these
  growth conditions

4
Preliminary grouping

• Oxidase negative
• Bile resistant
• Many carbohydrates fermented growth, acid, gas
• Ferment glucose some to acid (MR)
• some neutral (VP)
• Some ferment lactose coliforms (E. coli)
• Reduce nitrates to nitrites
• Catalase positive - facultative
• Motile, some by polar monotrichous flagella and
  some by peritrichous flagella. Few are
  non-motile all Shigella, some Klebsiella and
  some Salmonella(?)

5
Preliminary grouping

• The most frequently isolated enterics have been
  categorized into subgroups or tribes that share
  similar characteristics. These groupings are not
  scientifically legitimate, and are almost
  exclusively used for the enterics. Below NOT on
  test.
• Escherichieae Escherichia and Shigella
• Edwardsielleae Edwardsiella
• Salmonelleae Salmonella, Citrobacter
• Klebsielleae Klebsiella, Enterobacter, Serratia,
  and Hafnia
• Proteeae Proteus, Providencia, Morganella
• Yersinieae Yersinia

6
Cultural characteristics

• As mentioned previously, enterics are
  non-fastidious and bile-resistant, but fairly
  antibiotic sensitive typically inhibited on
  media with colistin, nalidixic acid, polymyxin or
  cefaperazone
• Most enteric selective media contain bile salts
  and/or dyes with bile-like properties
  (surfactants) that inhibit non-enteric bacteria
• MacConkey agar is the most commonly used medium
  of this type. It contains bile salts and crystal
  violet. Eosin-Methylene Blue (EMB) is an
  alternative to MacConkey. EMB contains eosin and
  methylene blue. SS agar (Salmonella Shigella)
  is similar to MacConkey but also indicates sulfur
  reduction.
• All of these media contain lactose and a pH
  indicator to differentiate lactose fermenters
  (yes or no and how much). Fecal coliforms such
  as E. coli produce acid from lactose fermentation
  resulting in darkened colonies on these media.

7
Cultural characteristics

• Even though the name SS makes this medium sound
  ideal, several literature reports state that some
  strains of Shigella fail to grow on it
• XLD and Hektoen are selective enteric agars used
  for primary plating and subculture of enrichment
  broths, and studies have shown that XLD and
  Hektoen are less likely to inhibit enteric
  species (produce higher yields than SS).
• Like SS, XLD and HE are also based on lactose
  fermentation and indicate sulfur reduction.
  Hydrogen sulfide production results in black
  colonies and on some media the agar becomes black
  as well.
• The H2S test component of these media is
  particularly helpful because Salmonella, Proteus,
  Edwardsiella, and an occasional Citrobacter
  produce hydrogen sulfide whereas most normal gut
  microbiota do not. Salmonella Edwardsiella are
  significant in this list as enteric pathogens
• All Shigella species are H2S-negative

8
Cultural characteristics

• Enrichment media have been used for enterics in
  the past. Using selenite broth allows
  Salmonella (resistant), which is often a distinct
  minority in the gut of carriers, to quickly
  outgrow the other normal gut microbiota.
• After an incubation time of 6 to 12 hours,
  selenite broth is subcultured to a selective
  agar, greatly enhancing the likelihood of
  isolating Salmonella in carriers
• Selenite broth may not be needed in acutely ill
  patients because Salmonella will likely be the
  predominate gut bacterium
• Another enrichment broth medium, GN is claimed by
  the manufacturer to enrich for both Salmonella
  and Shigella. To be effective GN must be
  subcultured after only 4 to 6 hours of
  incubation. Why?

9
Cultural characteristics

• Toxigenic E. coli strains have the same cultural
  characteristics as commensal strains of E. coli
  making the former difficult to detect
• One exception is verotoxin (shiga) producing E.
  coli (VTEC), a.k.a enterohemorrhagic (EHEC) E.
  coli O157H7
• The majority of these strains do not ferment the
  carbohydrate sorbitol whereas the normal strains
  usually do
• Sorbitol MacConkey (SMAC) contains the same
  ingredients as regular MacConkey except that
  sorbitol replaces lactose
• Commensal E. coli produce the same dark colonies
  on SMAC as on regular MacConkey but EHEC colonies
  are colorless
• Colonies suggestive of these strains are usually
  further tested for the toxin with commercially
  available ELISA tests

10
Identification

• Gram (-) rods that behave like enterics on Mac,
  EMB, etc should be screened with the oxidase
  test. All enterics are oxidase (-)
• Few non-enterics resemble the enterics and are
  oxidase (-), and these are easily distinguished
  with further testing
• Cultures with typical enteric morphology on Mac,
  are oxidase (-), and are indole () presumptively
  ID as E. coli with a high probability. The last
  2 tests must be run on cells from a medium devoid
  of pH indicators SBA colonies normally used.
• Proteus has the distinctive swarming
  morphology. Over 95 of Proteus isolates from
  clinical specimens is Proteus mirabilis, which is
  the only frequently isolated Proteus that is
  indole (-) making this an easy organism to ID.
• P. penneri is also indole (-), however it is
  extremely rare in clinical specimens and can be
  differentiated from P. mirabilis based upon
  different antimicrobial sensitivity patterns
• The occasionally isolated P. vulgaris is indole
  ().

11
Identification

• Before the advent of semi-automated and automated
  ID systems certain enterics were presumptively
  identified by the small battery of biochemical
  tests indole, methyl red, Voges Proskauer, and
  Citrate, the so-called IMViC series
• Some labs still use IMViC, so questions about it
  may still appear on certification examinations.
  Below NOT on test, unless
• Escherichia coli IMViC - -, lac
• Shigella IMViC - - - or - -, lac-,
  non-motile
• Klebsiella/Enterobacter IMViC - - , lac
  (Klebsiella nonmotile)
• Serratia IMViC - - , DNAse , lac variable
• Salmonella IMViC - - , lac-, H2S, lysine
  decarboxyase
• Citrobacter IMViC- - or - , lac
  (mostly), H2S, lysine decarboxyase -

12
Identification

• Oxidase (-) Gram (-) rods isolated from normally
  sterile sites such as blood and spinal fluid
  should be completely identified
• Isolates that appear to be enterics and are
  oxidase (-) should be tested for glucose
  fermentation all enterics ferment glucose.
• Kliglers iron agar (KIA), Triple Sugar Iron (TSI)
  agar, O/F glucose, glucose fermentation broth and
  other media can be used for this purpose. All
  are based upon a pH indicator color change via
  acid from glucose.
• KIA and TSA, the most commonly used media for
  this purpose in clinical labs, are deep slants
  containing phenol red as a pH indicator. To
  inoculate you stab the butt and streak the
  slant.
• The development of a yellow (acid) butt with a
  red (alkaline) slant after 12-24h of 35oC
  incubation indicates that the organism ferments
  glucose only (K/A is shorthand is for alkaline
  slant, acid butt)

13
Test for Fermentation of Glucose
Kliglers or Triple Sugar Iron (K/A)
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
Acid butt, alkaline slant (K/A) indicates
fermentation of glucose only
14
Test for Fermentation of Glucose
Kliglers or Triple Sugar Iron (A/A)
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
Acid throughout indicates fermentation of glucose
and lactose
15
Test for Fermentation of Glucose
Kliglers or Triple Sugar Iron (K/K or NC)
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
No acid indicates non-fermenter Rules out enteric
16
Identification

• Hugh and Leifsons (HL) oxidation/fermentation
  (O/F) medium contains a pH indicator (Bromthymol
  blue - BTB), peptone and a carbohydrate of your
  choice. BTB is green when neutral, blue when
  alkaline, and yellow when acid
• Two tubes of HL glucose media are stabbed to the
  butt with a needle and 1 of the tubes is overlaid
  with sterile mineral oil to prevent O2 diffusion
  this is the F tube.
• An O reaction is indicated by a yellow O tube
  and no change in the F tube (yellow in the O tube
  occurs due to CO2 from respiration reacting with
  water to make carbonic acid). An F reaction is
  indicated by yellow in both tubes these
  organisms are glucose fermentors, such as
  enterics
• Blue in either tube is an indication of amine
  accumulation from peptone utilization, useful
  data for some Gram (-) non-fermenting (GNNF)
  organisms HL is better for GNNFs than KIA or
  TSI

17
Test for Utilization of Glucose
Fermentation in Hugh and Leifsons test
Stab inoculate with a needle
Incubate 35oC 12-24h
Acid below the oil indicates glucose fermentation
No oil
Oil
18
Test for Utilization of Glucose
Oxidation in Hugh and Leifsons test
Stab inoculate with a needle
Incubate 35oC 12-24h
Acid in top of open tube only indicates
oxidation of glucose This rules out enterics
No oil
Oil
19
Test for Utilization of Glucose
Nonfermenter , Non-oxidizer (Inert)
Stab inoculate with a needle
Incubate 35oC 12-24h
No oil
No acid in either tubenonfermenter/nonoxidizer
This rules out enterics
Oil
20
Screening for Enteric Pathogens

• Enteric isolates from stool specimens can be
  screened with certain media/tests to eliminate
  some possibilities. This makes the final ID
  easier, but the down side is that it takes time.
• Salmonella, Shigella, and Edwardsiella are
  enteric pathogens that produce clear and
  colorless colonies on enteric agars.
• The Proteus group (Proteus, Providencia, and
  Morganella), and a few others, are not enteric
  pathogens. However, these may ( with exceptions)
  also produce colorless colonies on enteric agars
• All colorless colonies growing on enteric agars
  could be subjected to a complete battery of tests
  but this would not be practical since over 90 of
  non-lactose fermenters isolated from feces are
  not the 3 enteric pathogens mentioned above.
• The use of a few tests is effective for ruling
  out Salmonella Shigella before using an
  expensive automated or semi-automated product /
  approach.

21
Screening for Enteric Pathogens

• KIA (or TSI), Lysine Iron Agar (LIA) and
  Christensens Urea agar are media used for
  screening enteric pathogens.
• The fermentation/pH and the H2S reactions for the
  enteric bacteria as a group were discussed in
  previous slides in this series.
• Remember that Salmonella, Shigella and
  Edwardsiella produce alkaline slants and acid
  butts in KIA or TSI, and that Salmonella
  Edwardsiella are sulfur positive but Shigella is
  sulfur (-)
• Proteus species and Citrobacter fruendii are
  usually also H2S positive but they are not
  enteric pathogens
• Organisms that give any other reaction pattern on
  these media are not enteric pathogens
• See KIA or TSI reaction patterns on following
  slides

22
Screening for Enteric Pathogens
Kliglers Reactions (K/A)
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
Shigella cannot rule out
23
Screening for Enteric Pathogens
Kliglers Reactions (K/A)
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
Salmonella Edwardsiella cannot be ruled out
24
Screening for Enteric Pathogens
Kliglers Reactions (A/A)
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with cap loose
Not an enteric pathogen (typical of E.coli,
Klebsiella , or Enterobacter)
25
Screening for Enteric Pathogens
Kliglers Reactions (A/A)
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with cap loose
Not an enteric pathogen (typical of most
Citrobacter species)
26
Screening for Enteric Pathogens
Kliglers Reactions (K/NC)
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
No acid in butt rules out all enterics including
enteric pathogens, typical of Pseudomonas and
Acinetobacter
27
Screening for Enteric Pathogens

• LIA is also a deep slant, and is inoculated as is
  KIA TSI
• LIA contains sulfur iron, the amino acid
  lysine, a small amount of glucose, and the pH
  indicator Bromcresol Purple (BCP) which is purple
  if alkaline and yellow if acidic
• The by-product of lysine deamination (occurs near
  the top - aerobic) reacts with the iron causing a
  color change from purple to red.
• Only the Proteus group (Proteus species,
  Morganella morganii, and Providencia species)
  cause this reaction. This is important because
  the Proteus group mimics enteric pathogens
• Lysine decarboxylation (occurs where O2 is
  low) forms alkaline amines. Enteric bacteria
  growing in the butt will ferment the glucose, but
  lysine decarboxylation neutralizes the acid from
  fermentation and turns the butt purple.
  Presencf of growth distinguishes this from NC.
  Salmonella is lysine decarbox ().
• A yellow butt reaction is typical of Shigella
  species
• Keep in mind that Proteus, Salmonella and
  Citrobacter produce the black color via sulfur
  reduction as well as the above results

28
Screening for Enteric Pathogens
Lysine Iron Agar (K/K) Lysine decarb H2S
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
Typical reaction Salmonella species
29
Screening for Enteric Pathogens
Lysine Iron Agar (Red/A) Lysine deam H2S
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
Typical reaction of Proteus
30
Screening for Enteric Pathogens
Lysine Iron Agar (K/A) NC on slant, acid butt
H2S
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
Typical reaction of some Citrobacter species
31
Screening for Enteric Pathogens
Lysine Iron Agar (K/A)
Stab inoculate with a needle and streak the slant
Incubate 35oC 12-24h with loose cap
Typical reaction of Shigella and Some E. coli
32
Screening for Enteric Pathogens

• Christensens urea agar (another deep slant)
  contains glucose, a pH buffer, urea, and
  phenolphthalein as a pH indicator.
• Urease produced by non-enteric pathogens growing
  on this medium changes the urea to ammonia
• Depending on the amount of ammonia produced, the
  phenolphthalein color change ranges from a very
  pale pink to darker pink or a deep fuschia
• Salmonella and Shigella grow but do not produce
  urease the medium remains pale
• A positive urease rules out Salmonella and
  Shigella
• The Proteus group (Proteus, Morganella
  Providencia) are urease positive

33
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34
Screening for Enteric Pathogens
Christensens Urea Agar (Urease )
Streak the slant
Incubate 35oC 12-24h with loose cap
Typical reaction of Salmonella and Shigella
species (as well as E. coli )
35
Screening for Enteric Pathogens
Christensens Urea Agar (Urease weakly )
Streak the slant
Incubate 35oC 12-24h with loose cap
Typical reaction of Citrobacter, Serratia, and
Klebsiella species rules Salmonella and
Shigella
36
Screening for Enteric Pathogens
Christensens Urea Agar (Urease strongly )
Streak the slant
Incubate 35oC 12-24h with loose cap
Typical reaction of Proteus species Rules out
Salmonella and Shigella
37
Summary KIA and LIA Screen for enteric pathogens
KIA Reactions X on test
LIA React. R/A K/K K/K K/A K/A
K/A- Prot. d Sal. a Sal. a Citr. d
K/K Pseud. c d
K/A- Prot. d Sal. a Edwr a. Edwr. a . Sal. a
b Sal. a b
K/A-- Morg. d Prov. d Sal. a Pleis. c Aerom.
a Shig. a
K/A Prot. d Sal. a Sal. a
Sal. a b Citr. d Citr. d
A/A- E. coli d Kleb. d E. coli d
Kleb. d Enter. d
A/A-- Serrat. d
a Indicates enteric pathogen, Sal. and Shig.
should be serotyped, b Indicates unusual
reaction, cOxidase positive d Not enteric
pathogen
38
Serotyping

• Isolates presumptively identified as Salmonella
  or Shigella should be confirmed by serotyping.
• Isolates identified as Edwardsiella are not
  usually serotyped but are sent to a reference lab
  as this is an enteric pathogen
• Enteropathogenic E. coli (diarrhea, etc) can also
  be serotyped
• Enterics have three types of antigens which can
  be detected by agglutination tests O (somatic),
  K or Vi, and H antigens.
• Somatic (of the body or of the body wall) or O
  antigens are heat stable and are part of the Gram
  (-) bacterial cell wall
• K or Vi antigens are heat labile and, if
  present, represent the bacterial capsule. These
  are the most external antigens, and therefore can
  mask the presence of O antigens (ie. will not
  show up on agglutination test). K is for the
  German Kapsule, and Vi is derived from the word
  envelope.

39
Serotyping

• H antigens, if present, are also heat labile
  and are part of the bacterial flagella
• Since K and H antigens are heat labile they can
  be removed by boiling the culture for 15 min.
• After boiling, cultures are tested for somatic
  antigens. Somatic antigen is the primary antigen
  used to group (serological groups or serotypes)
  Salmonella Shigella isolates via antigenic
  relatedness
• 95 of human clinically significant Salmonella
  belong to the somatic groups A through G
• S. typhi belongs to group D, and S. paratyphi to
  group C
• Reference labs serotype these isolates using K
  and H antigens as well for confirmation. Most S.
  typhi strains and a few paratyphoid biotypes
  also posses the K antigen.

40
Serotyping

• Three types of antigens possessed by enterics

H
41
Serotyping

• Shigella species are divided into four somatic
  groups, A through D, each corresponding to a
  different species S. dysenteriae (A), S.
  flexneri (B), S. boydii (C), and S. sonnei (D)
• Any Shigella species may also possess K antigens,
  but since they are nonmotile they never posses H
  antigens
• Enteropathogenic E. coli are relatively hard to
  detect serologically, but most can be serotyped
  similarly to Salmonella and Shigella
• E. coli O157H7 is a well known serotype but
  there are others
• Serotyping each E. coli isolated from feces is
  impractical
• Detecting them by culture is a problem because
  most toxigenic E. coli have the same cultural
  characteristics as indigenous colon E. coli, but
  we did discuss a few ways to do this earlier -
  SMAC
• Some labs make available to the physician
  serological procedures that test for the various
  toxins produced by E. coli directly from the
  stool specimen

42
Serotyping

• The LGH Micro lab evaluated the following
  approach
• For 1 year a direct ELISA test for the verotoxin
  (shiga) was run on every stool received for
  enteric pathogen culture only three were found
  to be positive in the 12-month period
• Since each test cost about 10 the procedure was
  deemed not to be cost effective
• The current policy of the LGH Micro lab is to
  culture all stools on SMAC and test only
  colorless colonies for the Shiga toxin with an
  ELISA procedure
• This is not optimum because some EHEC strains
  have been shown to be sorbitol positive.
  Additionally, this procedure does not deal with
  other enteropathogenic E. coli serotypes
• Until research finds cost effective ways of
  detecting all EHEC and other enteropathogenic E.
  coli, the LGH method will have to suffice

43
Clinical Significance

• Enterics that cause primary intestinal infections
  include typhoidal and non-typhoidal Salmonella
  species (S. enteritidis), toxigenic strains of E.
  coli, Shigella species, and Yersinia
  entercolitica
• Enterics that do not cause intestinal infections
  can be associated with several extraintestinal
  infections as discussed previously - includes
  UTIs, LRT infections, GI tract, systemic, etc
• 90 of modern non-intestinal enteric infections
  are caused by only three species E. coli, K.
  pneumoniae, and P. mirabilis
• See following table for summary of common disease
  associations

44
Enterics and Common Infections They Produce
Bacterial Species
Diseases E. coli UTI,
septicemia, neonatal sepsis,
diarrheal syndromes Shigella Diarrhea,
dysentery Edwardsiella Diarrhea, wound
infections, enteric fever Citrobacter Oppor
tunistic and nosocomial infections (wounds,
UTI)
Slides 44-47 not on test --------?
45
Enterics and Common Infections They Produce
Bacterial Species
Diseases Klebsiella UTI, pneumonia, septicemia,
wounds Enterobacter Opportunistic and
nosocomial infections (UTI and
wounds) Serratia Opportunistic and
nosocomial infections (wound,UTI,
sepsis) Proteus UTI, wound infection, sepsis
46
Enterics and Common Infections They Produce
Bacterial Species
Diseases Providencia and Opportunistic and
nosocomial Morganella infections (UTI, wound,
sepsis) Yersinia pestis Bubonic and
pneumonic plague Yersinia entercolitica Diarrhea,
mesenteric lymph- adenitis (mimicking
appendicitis) Salmonella Diarrhea, enteric
fever, septicemia
47
Intestinal Illness caused by E. coli
Virulence Factor not known for sure
heat labile and stable enterotoxin direct
penetration spreadingfactors Shiga-like toxin,
verotoxin, strongly adhere to mucosa

Category
Major Symptoms infantile diarrhea, fever,
vomiting travelers diarrhea, cramps
dysentery,bloody stool, cramps, fever bloody
diarrhea, colitis, HUS diarrhea,
vomiting abdominal pain
Enteropathogenic (EPEC) Enterotoxigenic (ETEC) En
teroinvasive (EIEC) Enterohemorrhagic (EHEC)
(STEC) Enteroadherent (EAEC)
48
Virulence Factors

• All Gram-negative bacteria (not only pathogens)
  possess lipopolysaccharide (LPS) in the outer
  membrane of their cell envelope (membrane(s)
  wall)
• During an infection, the envelope can be degraded
  by phagocytic cells or due to antibiotic therapy.
  LPS can also be introduced in injectable drugs,
  IV solution, etc.
• Fragments of the lipid A portion of the LPS
  function as endotoxin in the body. Compared to
  exotoxins, endotoxin is heat stable, relatively
  (generally) mild in its affect, and less specific
  in its affect.
• Endotoxin stimulates phagocytic cells to release
  interleukin-1 (IL-1, formerly known as
  endogenous pyrogen). Il-1 travels to the
  hypothalamus via the blood circulation triggering
  an increase in body temperature (fever)

49
Virulence Factors

• Il-1 also stimulates phagocytes to release
  another cytokine, tumor necrosis factor (TNF) or
  cachectin (ku-kek-tin)
• This substance binds to tissues in the body
  causing damage. One example is TNF causing
  damage to blood capillaries causing them to
  become leaky. Severe cases are sufficiently
  acute to result in septic shock. A massive dose
  of endotoxin would be required for such as this
  it is very rare.
• Endotoxin can cause GI pathology which is usually
  mild
• Endotoxin can also cause diffuse intravascular
  coagulation
• Typhoid fever is a prime example of a disease in
  which endotoxin causes a significant amount of
  the pathology

50
Virulence Factors a few examplesX on test

• Klebsiella pneumoniae large capsule that
  interferes with phagocytosis and aids in alveolar
  attachment
• Shigella Shiga-like toxin, a very potent
  enterotoxin
• enteropathogenic E. coli verotoxin, surface
  protein for adherence to intestinal mucosa
• Proteus Urease, produces a large quantity of
  ammonia in the kidney during upper UTI which is
  toxic to renal parenchyma
• Salmonella direct invasion of mucosa,
  anti-phagocytosis, enterotoxigenic, those that
  produce enteric fever invade the blood stream
• Yersinia directly invade mucosa,
  anti-phagocytosis, enterotoxin

51
Antimicrobial susceptibility X on test

• Susceptibility to antibiotics varies
• Most E. coli involved in community acquired UTI
  are susceptible to many antibiotics. Most
  hospital strains become resistant to a myriad
  of antibiotics
• K. pneumonia is almost always resistant to
  ampicillin
• Enterobacter are resistant to first generation
  cephalosporins
• Proteus mirabilis is intrinsically resistant to
  nitrofurantoin and tetracycline, both frequently
  used for treating UTI
• Providencia, Morganella, and Serratia are
  multi-resistant - often plasmid mediated and can
  easily be passed to other enterics via
  conjugation
• Enteric isolates thought to be causing an
  infection are almost always tested in vitro with
  a large battery of antimicrobial agents

52
Intestinal Diseases Caused by Non Enterics

• There are several bacteria that cause intestinal
  infections other than the enterics. Some grow on
  the enteric media used for primary isolation from
  clinical specimen (ex. MacConkey agar), and they
  often produce colonies indistinguishable from
  enterics
• The Vibrio group (Vibrio, Aeromonas, and
  Pleisiomonas) is one of these, and must be
  differentiated from the enterics
• Members of the Vibrio group are oxidase positive,
  a fact allowing quick differentiation from
  enterics
• Cells from a sweep of a primary isolation plate
  to collect cells from each representative colony
  can be tested for oxidase. If the test is
  oxidase positive, each colony is them tested
  individually to find the positive colony, and
  additional testing can be conducted.