5.2 Medically Important Gram-Negative Bacilli: Enterobacterales and Non-Fermenters

Medically Important Gram-Negative Bacilli: Enterobacterales and Non-Fermenters

Gram-negative bacilli constitute the dominant bacterial etiologies of healthcare-associated infections, enteric diarrheal illnesses, urosepsis, and septic shock. For the clinical medical technologist, distinguishing among members of the order Enterobacterales and differentiating them from non-fermentative Gram-negative bacilli (NFGs) requires an understanding of carbohydrate utilization kinetics, enzymatic pathways, selective agar indicators, and phenotypic reaction batteries. Mastery of these diagnostic frameworks is a core pillar of the Philippine Medical Technologists Licensure Examination (MTLE).

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1. Taxonomic Foundations and Core Characteristics of Enterobacterales

The order Enterobacterales comprises a vast group of ubiquitous, non-spore-forming Gram-negative bacilli or coccobacilli that colonize human and animal intestinal tracts, soil, and aquatic environments. Clinical members share the following foundational physiological characteristics:

1. Glucose Fermentation: All species ferment D-glucose to produce acid end products, with or without the production of hydrogen ($H_2$) and carbon dioxide ($CO_2$) gas.
2. Cytochrome Oxidase Negative: They lack the cytochrome $c$ oxidase enzyme. When smeared on filter paper saturated with 1% tetramethyl-$p$-phenylenediamine dihydrochloride, no dark purple indophenol blue color develops within 10 to 30 seconds.
   • The Notable Exception: Plesiomonas shigelloides (formerly assigned to Vibrionaceae, now reclassified within Enterobacterales) is uniquely cytochrome oxidase positive.
3. Nitrate Reduction: All species possess the enzyme nitrate reductase, which reduces nitrate ($NO_3^-$) to nitrite ($NO_2^-$). Reduction is detected by the addition of sulfanilic acid (Reagent A) and $\alpha$-naphthylamine (Reagent B), yielding a red diazonium dye. If no color appears, zinc dust is added; development of red color indicates unreduced nitrate (a true negative test), whereas no color change after zinc indicates complete reduction past nitrite to nitrogen gas ($N_2$).
4. Facultatively Anaerobic: They grow robustly under both atmospheric aerobic conditions and anaerobic environments.
5. Motility: Most species are motile via peritrichous flagella. The classical non-motile exceptions are Klebsiella, Shigella, and Yersinia (which is uniquely temperature-dependent: non-motile at $37^\circ\text{C}$, but motile via peritrichous flagella at ambient room temperature, $22\text{--}25^\circ\text{C}$).

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2. Biochemical Identification Systems and Reaction Principles

Triple Sugar Iron (TSI) Agar

Triple Sugar Iron (TSI) agar is a foundational differential slant medium that assesses an organism's capacity to ferment specific carbohydrates and liberate hydrogen sulfide gas.

                               Triple Sugar Iron (TSI) Formulations
                                ┌─────────────────────────────────┐
                                │  0.1% Glucose (1 part)          │
                                │  1.0% Lactose (10 parts)        │
                                │  1.0% Sucrose (10 parts)        │
                                │  Phenol Red (pH indicator)      │
                                │  Ferrous Sulfate / Thiosulfate  │
                                └────────────────┬────────────────┘
                                                 │
         ┌───────────────────────────────────────┼──────────────────────────────────────┐
         ▼                                       ▼                                      ▼
    K / A (Red/Yellow)                      A / A (Yellow/Yellow)                  K / K (Red/Red)
• Glucose fermented only                • Glucose AND Lactose/Sucrose          • No sugars fermented
• 0.1% glucose exhausted in 8-12h         fermented                            • Obligate aerobe or
• Slant reverts to alkaline (red)       • Sustained acid maintains               non-fermenter
  via oxidative peptone deamination       entire tube yellow                   • Peptones deaminated
• Anaerobic butt remains acid           • Common: E. coli, Klebsiella,           aerobically on slant
• Common: Shigella, Salmonella,           Enterobacter                         • Common: Pseudomonas
  Proteus, Morganella

Slant / Butt Dynamics

• Inoculation Technique: Inoculate with a straight needle by stabbing the center of the deep butt to within 3 to 5 mm of the bottom, then streaking the surface of the slant with a serpentine motion. Incubate aerobically with a loose cap at $35^\circ\text{C}$ for 18 to 24 hours.
• Alkaline Slant / Acid Butt ($K/A$, Red/Yellow): Indicates glucose fermentation only. Because glucose is present at a low concentration ($0.1\%$, one-tenth that of lactose and sucrose), it is completely exhausted within 8 to 12 hours. The organism on the slant surface then catabolizes peptones oxidatively in the presence of oxygen, releasing alkaline amine byproducts (ammonia, $\text{NH}_3$) that neutralize the acid and shift the phenol red indicator back to red (alkaline, $K$). In the anaerobic butt, low oxygen tension prevents oxidative deamination; fermentative acid end products persist, maintaining a yellow butt (acid, $A$).
• Acid Slant / Acid Butt ($A/A$, Yellow/Yellow): Indicates fermentation of glucose plus lactose and/or sucrose. High concentrations of lactose ($1.0\%$) and sucrose ($1.0\%$) provide an abundant carbohydrate substrate. Continuous acid production overwhelms any alkaline amines generated from peptone breakdown, keeping both slant and butt yellow.
• Alkaline Slant / Alkaline Butt ($K/K$, Red/Red or No Change): Non-fermentative Gram-negative bacillus (e.g., Pseudomonas aeruginosa). Carbohydrates are not catabolized fermentatively; peptone utilization produces localized alkaline drift.
• Gas Production: Cleaving of the agar column, cracks, or bubbles caused by $CO_2$ and $H_2$ gas release during glucose fermentation.
• Hydrogen Sulfide ($H_2S$) Production: Sodium thiosulfate serves as a sulfur substrate. In an acidic environment, bacterial thiosulfate reductase releases colorless $H_2S$ gas, which reacts with the iron salt in the medium (ferrous sulfate in TSI; ferric ammonium citrate in some formulations) to yield an insoluble black precipitate of ferrous sulfide ($FeS$):

$\text{H}_2\text{S} + \text{FeSO}_4 \rightarrow \text{FeS} \downarrow \text{(Black)} + \text{H}_2\text{SO}_4$

• Kligler Iron Agar (KIA) versus TSI: KIA contains 0.1% glucose and 1.0% lactose, but completely omits sucrose. Certain organisms that ferment sucrose but not lactose (such as Proteus vulgaris) yield an $A/A$ reaction on TSI, but display a true glucose-only $K/A$ reaction on KIA.

The IMViC Battery

The classical IMViC series (Indole, Methyl Red, Voges-Proskauer, Citrate) provides essential separation among the coliforms:

1. Indole Test: Quantifies the ability of the enzyme tryptophanase to deaminate the amino acid tryptophan into indole, pyruvic acid, and ammonia. Bacteria are inoculated into 1% tryptone broth. Following 24-hour incubation, Kovac reagent ($p$-dimethylaminobenzaldehyde dissolved in isoamyl alcohol and concentrated hydrochloric acid) is added. Indole extracts into the upper solvent layer, reacting with the aldehyde to form an immediate cherry-red ring at the meniscus. (The alternative Ehrlich reagent uses ethyl alcohol and is preferred for fastidious anaerobes following xylene extraction).
2. Methyl Red (MR) Test: Assesses the mixed acid fermentation pathway. Organisms ferment glucose to produce large quantities of stable mixed organic acids (lactic, acetic, succinic, and formic acids), overcoming the phosphate buffer system in MR-VP broth and dropping the terminal pH below 4.4. Addition of methyl red indicator yields an immediate bright red color (positive). Yellow coloration indicates a terminal pH $> 6.2$ (negative).
3. Voges-Proskauer (VP) Test: Evaluates the alternative 2,3-butanediol fermentation pathway. Organisms convert glucose into pyruvate and subsequently to the neutral non-acidic intermediate acetoin (acetylmethylcarbinol). To 1 mL of 48-hour MR-VP broth, reagents are added sequentially: Barritt's Reagent A (5% $\alpha$-naphthol, acts as a color intensifier/catalyst) followed by Barritt's Reagent B (40% potassium hydroxide, KOH, the oxidizing agent). In the presence of atmospheric oxygen and alkali, acetoin is oxidized to diacetyl, which condenses with guanidine groups from peptone to produce a pink-to-ruby-red color within 15 to 30 minutes. MR and VP pathways are generally mutually exclusive among Enterobacterales.
4. Citrate Utilization Test: Evaluates whether an organism can utilize sodium citrate as its sole source of carbon and ammonium dihydrogen phosphate as its sole source of nitrogen. Formulated on Simmons citrate agar slants containing the pH indicator bromothymol blue. When citrate-permease-positive organisms transport and metabolize citrate, ammonium salts are cleaved to release alkaline ammonia ($\text{NH}_3$) and ammonium hydroxide, shifting the pH above 7.6. The medium shifts from its initial forest green to an intense Prussian blue. Growth on the slant accompanied by blue color is positive.

Phenylalanine Deaminase (PAD) Test

Detects the oxidative deamination of phenylalanine to phenylpyruvic acid:

$\text{Phenylalanine} + \text{H}_2\text{O} + \text{O}_2 \xrightarrow{\text{Phenylalanine Deaminase}} \text{Phenylpyruvic Acid} + \text{NH}_3 + \text{H}_2\text{O}$

Following overnight growth on a phenylalanine agar slant, 4 to 5 drops of 10% ferric chloride ($FeCl_3$) are added over the growth. Phenylpyruvic acid chelates ferric ions, generating an immediate, intense dark emerald green color on the slant surface. PAD positivity is the defining diagnostic hallmark of the PPM tribe: Proteus, Providencia, and Morganella.

Urease Test (Christensen's Urea Agar / Stuart's Broth)

Hydrolyzes urea to release carbon dioxide and two molecules of alkaline ammonia, shifting the phenol red indicator to a brilliant magenta / hot pink:

$\text{Urea} + 2\text{H}_2\text{O} \xrightarrow{\text{Urease}} 2\text{NH}_3 + \text{CO}_2$

• Rapid Urease Positive (within 2 to 4 hours): Proteus, Providencia rettgeri, Morganella morganii.
• Slow / Delayed Urease Positive (18 to 24 hours): Klebsiella pneumoniae, Enterobacter spp., Serratia marcescens.

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3. Major Enteric Pathogens and Coliforms

                                Enterobacterales Differentiation
                                               │
                                     Lactose Fermentation
                                     (MacConkey Agar, MAC)
                                               │
                   ┌───────────────────────────┴───────────────────────────┐
                   ▼                                                       ▼
            Lactose Fermenters                                    Non-Lactose Fermenters
             (Pink on MAC)                                           (Colorless on MAC)
                   │                                                       │
             IMViC Profile                                           H₂S Production
         ┌─────────┴─────────┐                                   ┌─────────┴─────────┐
         ▼                   ▼                                   ▼                   ▼
      ++ --               -- ++                               Positive            Negative
     (E. coli)       (Klebsiella spp.,                     (Salmonella,        (Shigella spp.,
                     Enterobacter spp.)                     Proteus spp.)       Yersinia spp.)

Escherichia coli

Escherichia coli is the predominant facultative anaerobe of the human colon, a leading agent of Gram-negative bacteremia, and the primary cause ($>80\%$) of uncomplicated urinary tract infections.

• Colony Characteristics: Robust lactose fermenter producing flat, dark pink-to-rose colonies on MacConkey agar surrounded by a precipitation zone of precipitated bile salts. On Eosin Methylene Blue (EMB) agar, rapid lactose fermentation precipitates eosin-methylene blue dyes, producing dark purple colonies with a distinctive greenish-metallic sheen.
• Biochemical Profile: Classical IMViC reaction: $++--$ (Indole positive, Methyl Red positive, Voges-Proskauer negative, Citrate negative); motile; lysine decarboxylase positive; $H_2S$ negative; gas positive ($A/A + \text{gas}$ on TSI).
• Major Diarrheagenic Pathotypes:
  • Enterotoxigenic E. coli (ETEC): Causes traveler's diarrhea ("Montezuma's revenge") via plasmid-encoded heat-labile enterotoxin (LT) (stimulates adenylate cyclase $\to \uparrow cAMP$) and heat-stable enterotoxin (ST) (stimulates guanylate cyclase $\to \uparrow cGMP$), causing hypersecretion of water and chloride ions into the lumen.
  • Enteropathogenic E. coli (EPEC): Causes severe infantile diarrhea in developing nations. Employs bundle-forming pili to attach to enterocytes, effacing microvilli via the eaeA intimin gene.
  • Enteroinvasive E. coli (EIEC): Invades and destroys colonic epithelial cells, producing dysentery with fever, abdominal tenesmus, and scant bloody-mucoid stools containing sheets of polymorphonuclear leukocytes, clinically mimicking shigellosis.
  • Enterohemorrhagic E. coli (EHEC / STEC): The archetype is serotype O157:H7. Produces Shiga-like toxins 1 and 2 (Stx1, Stx2 / Verotoxins). The B subunit binds globotriaosylceramide ($Gb_3$) receptors, while the active A subunit cleaves the 28S rRNA of the host 60S ribosomal subunit, irreversibly halting cellular protein synthesis. Ingestion of undercooked ground beef or unpasteurized cider triggers severe hemorrhagic colitis and Hemolytic Uremic Syndrome (HUS)—the clinical triad of microangiopathic hemolytic anemia, severe thrombocytopenia, and acute renal failure.
  • Laboratory Screening on Sorbitol-MacConkey (SMAC) Agar: Approximately 95% of normal intestinal E. coli strains ferment D-sorbitol rapidly, yielding pink colonies on SMAC. In contrast, pathogenic E. coli O157:H7 does NOT ferment sorbitol within 24 hours, appearing as distinct colorless / translucent colonies. Non-sorbitol-fermenting colonies are confirmed by serological latex agglutination for the O157 somatic and H7 flagellar antigens, or via molecular multiplex PCR for stx1/stx2 genes.

Klebsiella pneumoniae

• Colony Characteristics: Non-motile, robust lactose fermenter ($A/A + \text{gas}$ on TSI). Synthesizes an expansive polysaccharide capsule that imparts a prominent, mucoid, viscous appearance on agar plates. When touched with an inoculating loop, colonies draw up into tenacious, stringy strands.
• Biochemical Profile: Classical IMViC reaction: $--++$ (Indole negative, Methyl Red negative, Voges-Proskauer positive, Citrate positive); strictly non-motile; urease positive; ornithine decarboxylase negative; lysine decarboxylase positive.
• Clinical Spectrum: Causative agent of severe, destructive community-acquired lobar pneumonia in alcoholics, diabetics, and chronic lung disease patients. Massive tissue necrosis and microabscess formation produce expectoration of viscous hemoptysis classically termed "red currant jelly" sputum. A major agent of catheter-associated UTIs, ventilator-associated pneumonia, and nosocomial bacteremia. Strains harboring plasmid-mediated Klebsiella pneumoniae carbapenemase (KPC) confer broad carbapenem resistance (CRE).

Serratia marcescens

• Biochemical Hallmarks: Late or slow lactose fermenter (frequently appears as a non-lactose fermenter on primary MAC plates within 24 hours because it possesses $\beta$-galactosidase but lacks $\beta$-galactoside permease; ONPG positive). Uniquely positive for a trio of active hydrolytic enzymes: DNase positive, gelatinase positive, and lipase positive.
• Pigment Production: Clinical and environmental strains produce a non-diffusible, bright magenta-red pigment called prodigiosin when incubated at room temperature ($20\text{--}25^\circ\text{C}$); pigment production is frequently suppressed at $35\text{--}37^\circ\text{C}$.
• Clinical Significance: Nosocomial opportunistic pathogen causing ventilator-associated pneumonia, intravenous catheter sepsis, and urinary tract infections in intensive care units; displays intrinsic resistance to ampicillin and early-generation cephalosporins.

Proteus Species (Proteus mirabilis and Proteus vulgaris)

• Colony Characteristics: Non-lactose fermenters. On non-inhibitory solid media (sheep blood agar, nutrient agar), cells undergo cycles of differentiation into elongated hyperflagellated swarmers, producing swarming motility characterized by progressive, concentric ripples of bacterial growth across the plate.
• Biochemical Markers: Rapid and potent urease positive (splits Stuart's broth within 2 to 4 hours); phenylalanine deaminase (PAD) positive; hydrogen sulfide ($H_2S$) positive; TSI reaction is $K/A + H_2S$.
• Clinical Impact: Urease activity alkalinizes urine (pH $> 8.0$), driving the precipitation of magnesium ammonium phosphate and calcium carbonate, forming massive struvite staghorn calculi that obstruct renal pelvocalyceal systems.
• Species Differentiation:
  • Proteus mirabilis: Indole negative, ornithine decarboxylase positive, susceptible to ampicillin.
  • Proteus vulgaris: Indole positive, ornithine decarboxylase negative, resistant to ampicillin.

Salmonella Species (Salmonella enterica)

• Biochemical Hallmarks: Non-lactose fermenters (colorless on MAC); motile by peritrichous flagella; $H_2S$ positive (forms colonies with dark black centers on selective Hektoen Enteric [HE] and Xylose Lysine Deoxycholate [XLD] agars); lysine decarboxylase positive; indole negative; TSI reaction: $K/A + \text{gas} + H_2S$.
• Salmonella enterica serovar Typhi: Causative agent of typhoid (enteric) fever. Characterized on TSI by producing only a trace amount of $H_2S$, classically visible as a delicate black crescent or collar at the slant-butt interface, with minimal gas production. Possesses the capsular Vi (virulence) antigen, which masks underlying somatic O antigens during agglutination testing (cleared by heating the bacterial suspension at $100^\circ\text{C}$ for 15 minutes). Diagnosed serologically via the historic Widal test, which measures rising agglutinin titers against somatic O antigen (indicating active infection) and flagellar H antigen.

Shigella Species

• Biochemical Hallmarks: Non-lactose fermenters; strictly non-motile; $H_2S$ negative; gas negative; lysine decarboxylase negative. TSI reaction: $K/A$ without gas or $H_2S$.
• Pathogenesis: Highly infectious with an extraordinarily low infectious dose (10 to 100 viable organisms) capable of surviving gastric acidity. Causes bacillary dysentery via enterocyte invasion, actin polymerization-mediated cell-to-cell spread, and mucosal microabscess formation.
• Serogroups (Based on Somatic O Antigen):
  • Group A: Shigella dysenteriae (Most severe; produces the potent neuro-cytotoxin Shiga toxin; mannitol negative).
  • Group B: Shigella flexneri (Mannitol positive).
  • Group C: Shigella boydii (Mannitol positive).
  • Group D: Shigella sonnei (Most common species in developed nations; slow / late lactose fermenter; uniquely ONPG positive and ornithine decarboxylase positive).

Yersinia enterocolitica

• Biochemical Hallmarks: Non-lactose fermenter; motile at $22\text{--}25^\circ\text{C}$, but strictly non-motile at $37^\circ\text{C}$; urease positive; sucrose fermenter ($A/A$ with yellow slant and butt on TSI due to 1% sucrose, mimicking a coliform).
• Selective Isolation: Exhibits enhanced recovery following cold enrichment (holding stool suspensions in phosphate-buffered saline at $4^\circ\text{C}$ for 1 to 3 weeks). On selective Cefsulodin-Irgasan-Novobiocin (CIN) agar incubated at $25\text{--}30^\circ\text{C}$, mannitol fermentation produces distinctive "bull's-eye" colonies featuring an intense dark-pink/red center surrounded by a transparent border.
• Clinical Presentation: Enterocolitis in young children and acute mesenteric lymphadenitis in adolescents, which presents with severe right lower quadrant abdominal tenderness mimicking acute appendicitis (pseudoappendicitis).

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4. Non-Fermentative Gram-Negative Bacilli (NFGs)

Non-fermenters are aerobic Gram-negative bacilli that do not utilize carbohydrates via anaerobic fermentation. In Hugh-Leifson Oxidative-Fermentative (OF) medium, they either oxidize glucose (acid in the open aerobic tube only) or are completely asaccharolytic (alkaline / green in both tubes).

Pseudomonas aeruginosa

Pseudomonas aeruginosa is the preeminent opportunistic non-fermentative pathogen in clinical medicine, ubiquitous in hospital water taps, respiratory therapy nebulizers, and sink drains.

• Phenotypic Criteria: Obligate aerobe; cytochrome oxidase positive; oxidizes glucose in OF open tube; motile via a single polar monotrichous flagellum.
• Colonial Morphology & Odor: Flat, spreading colonies with serrated margins and an iridescent metallic sheen on sheep blood agar. Emits a distinctive sweet, grape-like or corn-tortilla odor caused by 2-aminoacetophenone.
• Pigment Production: Produces two characteristic diffusible pigments:
  1. Pyocyanin: Blue, water-soluble, chloroform-soluble non-fluorescent phenazine pigment unique to P. aeruginosa; induces oxidative stress in host airway epithelial cells.
  2. Pyoverdine (Fluorescein): Yellow-green, water-soluble fluorescent pigment that fluoresces under ultraviolet light ($365\text{ nm}$); acts as an iron-chelating siderophore. The combination of pyocyanin and pyoverdine produces the vibrant blue-green coloration of colonies and purulent surgical drainage.
• Growth at $42^\circ\text{C}$: Capable of growing robustly at $42^\circ\text{C}$, a key physiological marker distinguishing it from other fluorescent pseudomonads (P. fluorescens, P. putida).
• Clinical Spectrum: Leading etiology of ventilator-associated pneumonia (VAP), hot tub folliculitis, malignant otitis externa in diabetics, burn wound sepsis, ecthyma gangrenosum (black necrotic cutaneous ulcers with erythematous halos in neutropenic patients), and endobronchial mucoid colonization in cystic fibrosis patients driven by massive alginate exopolysaccharide synthesis.

Acinetobacter baumannii

• Microscopic & Cultural Features: Plump, pleomorphic Gram-negative coccobacilli that appear in pairs, often misidentified as Neisseria on direct Gram stains. Strictly aerobic; cytochrome oxidase negative; strictly non-motile.
• Clinical Significance: Highly resistant nosocomial pathogen colonizing dry abiotic surfaces, mechanical ventilators, and central venous catheters in intensive care units. Often expresses extensive $\beta$-lactamases, aminoglycoside-modifying enzymes, and carbapenemases (CRAB: Carbapenem-Resistant Acinetobacter baumannii).

Stenotrophomonas maltophilia

• Biochemical Hallmarks: Straight Gram-negative bacillus; motile by polar multitrichous flagella; cytochrome oxidase negative; oxidizes maltose more rapidly than glucose; forms lavender-green colonies on sheep blood agar accompanied by an ammonia-like odor.
• Antimicrobial Susceptibility: Intrinsically resistant to all carbapenems (imipenem, meropenem) due to chromosomally encoded metallo-$\beta$-lactamase ($L1$). The therapeutic drug of choice is trimethoprim-sulfamethoxazole (TMP-SMX).

Burkholderia Species

• Burkholderia cepacia Complex: Cytochrome oxidase positive (or weak); non-wrinkled yellow colonies; oxidizes glucose; opportunistic pulmonary pathogen causing severe necrotizing pneumonia and "cepacia syndrome" in cystic fibrosis.
• Burkholderia pseudomallei: Causative agent of melioidosis, endemic in Southeast Asia (including rural agricultural areas of the Philippines) and northern Australia. Smears demonstrate Gram-negative bacilli with prominent bipolar safety-pin staining. Produces dry, wrinkled, rugose colonies on Ashdown medium with a characteristic earthy/musty odor. Tier 1 select biothreat agent.

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5. Consolidated Enterobacterales and Non-Fermenter Reaction Matrix

The following consolidated matrix outlines the definitive biochemical and enzymatic reactions for differentiating major Gram-negative bacilli: