5.4 Mycobacteria, Spirochetes, Atypical Bacteria, and Public Health Microbiology

Mycobacteria, Spirochetes, Atypical Bacteria, and Public Health Microbiology

Clinical bacteriology encompasses specialized microorganisms with unique envelope structures, obligate intracellular niches, and critical environmental vectors. Acid-fast bacilli, pathogenic spirochetes, and cell-wall-deficient organisms require specialized staining chemistries, cell cultures, and serological assays. Furthermore, the clinical medical technologist plays a pivotal role in public health surveillance by executing standardized bacteriological quality testing for municipal drinking water and commercial dairy supplies. These high-yield topics are heavily tested on the Philippine Medical Technologists Licensure Examination (MTLE).

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1. Mycobacterium tuberculosis Complex (MTBC) and Molecular Diagnostics

Cell Wall Architecture and Acid-Fastness

The genus Mycobacterium is defined by an extraordinary cell envelope wherein lipids constitute up to 60% of the total dry cell wall weight. The plasma membrane is enclosed by a peptidoglycan layer covalently bonded to arabinogalactan, which in turn esterifies high-molecular-weight mycolic acids—branched $\beta$-hydroxy fatty acids containing 60 to 90 carbon atoms. Interspersed throughout this lipid matrix are free lipids including lipoarabinomannan (LAM), trehalose dimycolate, and sulfatides.

                                Mycobacterial Cell Envelope
      [ Exterior ]  ══════════════════════════════════════════════════════
                    Free Lipids (Cord Factor / Sulfatides / Mycosides)
                    ──────────────────────────────────────────────────────
                    MYCOLIC ACIDS (Long-chain β-hydroxy fatty acids, C60-C90)
                    ──────────────────────────────────────────────────────
                    Arabinogalactan Complex
                    ──────────────────────────────────────────────────────
                    Peptidoglycan Backbone
      [ Cytosol ]   ──────────────────────────────────────────────────────
                    Plasma Membrane (Phospholipid Bilayer)

This hydrophobic, waxy barrier makes mycobacteria impermeable to conventional crystal violet Gram staining, appearing as "Gram-ghosts" (faint, unstained beaded rods). However, once intensely stained with lipophilic phenolic dyes, they resist decolorization by mineral acids and alcohol—the defining property of acid-fastness.

Acid-Fast Staining Methodologies

1. Ziehl-Neelsen (Classical Hot Method): Primary stain is carbolfuchsin (basic fuchsin dissolved in phenol). Heat is applied until steam rises, softening the waxy mycolic acid matrix and driving the dye into the cytoplasm. Decolorization is performed using 3% acid-alcohol (3 mL concentrated hydrochloric acid in 97 mL 95% ethanol). Counterstaining is performed with methylene blue. Acid-fast bacilli (AFB) appear as slender, slightly curved, beaded bright red / fuchsia rods against a light blue background.
2. Kinyoun (Cold Method): Employs a higher concentration of basic fuchsin and phenol with a wetting agent (tergitol), eliminating the requirement for heat steaming.
3. Fluorochrome Staining (Auramine-O / Rhodamine): Fluorescent dyes bind selectively to mycolic acids. Decolorized with 0.5% acid-alcohol and counterstained with potassium permanganate (quenches non-specific background fluorescence). Under fluorescence microscopy, AFB fluoresce bright yellow-gold or orange against a dark field. This enables rapid examination under lower magnification ($250\times$ to $400\times$), covering broader smear areas in less time.

Cultivation and Colonial Morphology of M. tuberculosis

Mycobacterium tuberculosis is an obligate aerobe with an extremely slow generation time (18 to 24 hours), requiring 2 to 6 weeks of incubation at $35\text{--}37^\circ\text{C}$ in an atmosphere of 5% to 10% $CO_2$.

• Culture Media:
  • Egg-Based Media: Lowenstein-Jensen (LJ) medium (contains whole eggs, potato flour, glycerol, and malachite green as an inhibitory agent to suppress contaminating Gram-positive bacteria). Also Petragnani (higher malachite green, for heavily contaminated specimens) and American Thoracic Society (ATS) media.
  • Agar-Based Media: Middlebrook 7H10 and 7H11 (chemically defined clear agar media enabling early visualization of microscopic microcolonies within 10 to 14 days).
  • Automated Liquid Broth Systems: BACTEC MGIT 960 (Mycobacteria Growth Indicator Tube) contains modified Middlebrook 7H9 broth with a ruthenium fluorescent oxygen sensor embedded in silicone at the tube bottom. Actively respiring mycobacteria consume dissolved oxygen, relieving oxygen quenching and triggering automated fluorescence detection within 7 to 14 days.
• Colonial Appearance: Colonies on LJ slants are non-pigmented, cream to buff-colored, raised, dry, and rough, with an irregular friable surface classically described as "cauliflower-like" or "breadcrumb-like".
• Serpentine Cording: Microscopic smears prepared from colonies or broth cultures reveal long, parallel, undulating cords of acid-fast bacilli termed serpentine cording. This arrangement is mediated by cord factor (trehalose 6,6'-dimycolate), a primary glycolipid virulence factor that inhibits phagosome-lysosome fusion and leukocyte migration.

Definitive Biochemical Identification of M. tuberculosis

Molecular Diagnostics: GeneXpert MTB/RIF

The GeneXpert MTB/RIF assay is an automated, cartridge-based nested real-time PCR platform endorsed worldwide by the WHO. Directly from raw or decontaminated sputum, the system lyses mycobacteria, extracts DNA, and amplifies the $\textit{rpoB}$ gene (encoding the RNA polymerase $\beta$-subunit) within 2 hours. Five overlapping molecular beacon probes detect wild-type sequences; mutations in the 81-base pair core region of $\textit{rpoB}$ confer clinical resistance to rifampicin—serving as a rapid surrogate marker for Multidrug-Resistant Tuberculosis (MDR-TB).

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2. Non-Tuberculous Mycobacteria (NTM) Runyon Classification and M. leprae

Non-tuberculous mycobacteria (NTM, also called MOTT: Mycobacteria Other Than Tuberculosis) are environmental saprophytes found in soil and water. In 1959, Ernest Runyon classified NTM into four distinct groups based on rate of colonial growth and the production of yellow-orange carotenoid pigments in the presence or absence of light.

                         Runyon Classification of NTM
                                      │
                            Growth Rate on Solid Media
                                      │
              ┌───────────────────────┴───────────────────────┐
              ▼                                               ▼
         Slow Growers                                    Rapid Growers
         (> 7 Days)                                      (≤ 7 Days)
              │                                               │
     Pigment Photoreactivity                                 Group IV
  ┌───────────┼───────────┐                         (M. fortuitum, M. chelonae,
  ▼           ▼           ▼                          M. abscessus; MacConkey +)
Group I    Group II    Group III
Photo-      Scoto-      Nonphoto-
chromogens chromogens  chromogens

The Four Runyon Groups

1. Group I: Photochromogens (Slow Growers): Colonies are non-pigmented (buff) when incubated in continuous darkness, but synthesize bright yellow-to-orange carotenoid pigments within 6 to 24 hours upon exposure to light and re-incubation.
   • Mycobacterium kansasii: Chronic cavitary pulmonary infection resembling tuberculosis.
   • Mycobacterium marinum: Grows optimally at lower temperatures ($30\text{--}32^\circ\text{C}$); causes ulcerative cutaneous granulomas ("swimming pool granuloma" or "fish tank granuloma") acquired through skin abrasions contacting contaminated water.
2. Group II: Scotochromogens (Slow Growers): Synthesize deep yellow-to-orange carotenoid pigments in both the dark and the light.
   • Mycobacterium scrofulaceum: Causes chronic granulomatous cervical lymphadenitis (scrofula) in young children.
   • Mycobacterium gordonae: Widely recognized as the "tap water bacillus"; ubiquitous in municipal water lines and laboratory reagents; non-pathogenic laboratory contaminant.
3. Group III: Nonphotochromogens (Slow Growers): Colonies remain buff, off-white, or pale beige regardless of light exposure.
   • Mycobacterium avium-intracellulare Complex (MAC): Composed of M. avium and M. intracellulare. Highly opportunistic; the leading cause of disseminated mycobacterial disease in advanced HIV/AIDS patients with $CD4$ counts $< 50/\mu\text{L}$; tellurite reduction positive.
   • Mycobacterium ulcerans: Produces mycolactone toxin; causative agent of necrotizing skin and soft-tissue ulcers (Buruli ulcer).
4. Group IV: Rapid Growers: Mature visible colonies appear on routine solid media within 7 days (often within 3 to 5 days). Growth resembles diphtheroids or micrococci.
   • Mycobacterium fortuitum, M. chelonae, M. abscessus: Cause surgical wound infections, post-injection abscesses, and corneal ulcers; characterized by growth on modified MacConkey agar without crystal violet and a positive 3-day arylsulfatase test.

Mycobacterium leprae (Hansen's Disease)

• Biology: Obligate intracellular bacterium displaying strict tropism for cooler peripheral tissues ($30\text{--}33^\circ\text{C}$), specifically infecting Schwann cells of peripheral nerves and dermal macrophages.
• Unculturability: M. leprae cannot be cultured on any artificial cell-free bacteriological media. It is maintained experimentally by propagation in the footpads of immunocompromised mice or in the nine-banded armadillo (Dasypus novemcinctus).
• Laboratory Diagnosis: Demonstrated in skin slit smears, nasal scrapings, or earlobe biopsies stained with the Fite-Faraco modified acid-fast stain (incorporates peanut oil/xylene to protect the fragile, low-lipid mycobacterial wall from being stripped).
• Immunological Spectrum:
  • Tuberculoid Leprosy: Strong cell-mediated immunity ($T_H1$ response); localized anesthetic skin hypopigmentation with thickened nerves; few bacilli present (paucibacillary); lepromin skin test is positive.
  • Lepromatous Leprosy: Defective cell-mediated immunity ($T_H2$ predominance); widespread erythematous macules, papules, facial tissue destruction ("leonine facies"), and saddle-nose deformity; massive numbers of intracellular acid-fast bacilli packed in lipid-laden macrophages (globi / lepra cells; multibacillary); lepromin skin test is negative.

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3. Medically Important Spirochetes: Treponema, Borrelia, and Leptospira

Spirochetes are slender, helically coiled, flexible unicellular bacteria. Their unique structural hallmark is the endoflagella (periplasmic flagella) located in the periplasmic space between the peptidoglycan cylinder and outer membrane sheath, mediating corkscrew rotational motility.

                                Medically Important Spirochetes
                                               │
                   ┌───────────────────────────┼───────────────────────────┐
                   ▼                           ▼                           ▼
         *Treponema pallidum*        *Borrelia burgdorferi*       *Leptospira interrogans*
      • Tight coils; tapered ends  • Loose, irregular coils     • Very tight coils; hooked ends
      • Cannot be cultured on      • Cultured in BSK medium     • Cultured in Fletcher/EMJH
        cell-free media            • Transmitted by Ixodes      • Transmitted via rodent urine
      • Causes Syphilis              ticks (Lyme disease)         in floodwaters
      • Serology: VDRL/RPR +       • Wright-Giemsa stained      • Causes Weil Disease
        FTA-ABS / TP-PA              blood smears (relapsing)   • Microscopic Agglutination (MAT)

Treponema pallidum subsp. pallidum (Syphilis)

• Morphology: Slender, tightly wound spirochete with 8 to 14 uniform spirals; invisible under brightfield microscopy; cannot be propagated on artificial cell-free media.
• Clinical Stages:
  • Primary Syphilis: Characterized by the hard chancre—a solitary, painless, indurated, clean-based ulcer at the site of inoculation accompanied by painless regional lymphadenopathy; highly infectious.
  • Secondary Syphilis: Systemic dissemination occurring 2 to 10 weeks later; features a generalized maculopapular rash involving the palms and soles, mucosal patches, and condylomata lata (broad, moist, flat, highly infectious papules in warm intertriginous skin folds).
  • Latent Syphilis: Asymptomatic phase with serological reactivity (Early latent $< 1\text{ year}$; Late latent $> 1\text{ year}$).
  • Tertiary Syphilis: Develops years to decades later; manifestations include chronic destructive gummas (granulomas in skin and bones), cardiovascular syphilis (aortitis, thoracic aortic aneurysm), and neurosyphilis (tabes dorsalis, general paresis, Argyll Robertson pupils).
• Diagnostic Laboratory Testing:
  • Darkfield Microscopy: Direct visualization of motile spirochetes displaying rapid rotation around their long axis and stiff flexion in serous exudate harvested from primary chancres or condyloma lata.
  • Non-Treponemal Serological Tests (VDRL and RPR): Detect reaginic antibodies (a mixture of IgM and IgG) directed against an alcoholic cardiolipin-lecithin-cholesterol antigen complex released from host tissue damage. The Venereal Disease Research Laboratory (VDRL) test requires microscopic examination of micro-flocculation. The Rapid Plasma Reagin (RPR) test incorporates specially prepared charcoal particles to permit macroscopic visual agglutination on a card without a microscope. Non-treponemal titers correlate directly with disease activity: titers rise during primary/secondary stages, and a four-fold (two-dilution) drop in titer verifies successful therapeutic response. VDRL is the only serological test approved by the CDC for diagnosing neurosyphilis on cerebrospinal fluid (CSF).
  • Biological False Positives (BFP): Non-treponemal tests are non-specific and prone to false positives caused by systemic lupus erythematosus (SLE), viral infections (EBV, hepatitis), advanced age, malaria, pregnancy, and autoimmune diseases.
  • The Prozone Phenomenon: In secondary syphilis with extraordinarily high antibody titers, antibody excess blocks lattice formation, causing false-negative non-treponemal results. Suspected specimens must be serially diluted.
  • Treponemal Serological Tests (FTA-ABS, TP-PA, TPHA, EIAs): Detect antibodies directed specifically against native Treponema pallidum antigens. In the Fluorescent Treponemal Antibody Absorption (FTA-ABS) test, patient serum is pre-absorbed with non-pathogenic Reiter treponemal extract to remove cross-reactive commensal antibodies, then reacted with fixed T. pallidum on slides and stained with fluorescein-conjugated anti-human globulin. Treponemal tests serve as confirmatory tests following a reactive non-treponemal screen; they remain reactive for life in $>85\%$ of patients regardless of treatment.

Borrelia Species

• Microscopic Staining: Unlike Treponema, Borrelia species are thicker with loose, irregular coils and can be readily stained and visualized under light microscopy with Wright-Giemsa or Romanowsky stains.
• Borrelia burgdorferi (Lyme Disease): Transmitted by the bite of the blacklegged deer tick (Ixodes scapularis). Stage 1 is marked by erythema migrans—an expanding annular erythematous skin lesion with central clearing classically described as a "bull's-eye rash". Stage 2 involves neurological (Bell's cranial nerve palsy) and cardiac (atrioventricular heart block) abnormalities. Stage 3 causes chronic, destructive Lyme arthritis. Can be cultured in Barbour-Stoenner-Kelly (BSK) liquid medium.
• Borrelia recurrentis (Epidemic Relapsing Fever): Transmitted by the human body louse (Pediculus humanus). Manifests as recurring cyclic paroxysms of high fever and chills driven by antigenic variation of Variable Major Proteins (VMPs) on its outer membrane. Diagnosed directly by visualizing abundant spirochetes circulating between erythrocytes on a Wright-Giemsa-stained peripheral blood smear collected during the febrile paroxysm.

Leptospira interrogans

• Morphology: Extremely thin, flexible, tightly coiled spirochete ($0.1\ \mu\text{m} \times 6\text{--}20\ \mu\text{m}$) featuring distinctive semi-circular hooked ends resembling question marks or umbrella handles.
• Epidemiology & Transmission: Major public health hazard in tropical nations including the Philippines, particularly following severe typhoons and monsoon rains. Pathogenic Leptospira persistently colonize the renal tubular lumen of wild rodents (rats) and domestic animals, shedding into urine. Humans are infected through occupational or accidental immersion of abraded skin or mucous membranes in contaminated floodwaters or wet soil.
• Clinical Spectrum: Biphasic illness progressing from an anicteric flu-like illness with conjunctival suffusion (dilated scleral vessels without purulence) to severe Weil Disease—the clinical triad of severe hepatic jaundice, acute oliguric renal failure, and hemorrhagic diathesis with diffuse pulmonary hemorrhage.
• Laboratory Diagnosis:
  • Cultured on specialized semisolid media containing 10% rabbit serum and fatty acids—specifically Fletcher's medium or EMJH (Ellinghausen-McCullough-Johnson-Harris) medium—incubated at $28\text{--}30^\circ\text{C}$ in the dark for up to 6 to 8 weeks. Organisms appear in blood and CSF during the first 7 to 10 days, and in urine after the second week.
  • Microscopic Agglutination Test (MAT): The reference gold-standard serological assay, measuring agglutination and lysis of live Leptospira serovar suspensions under darkfield microscopy.

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4. Atypical Cell-Wall-Deficient and Obligate Intracellular Bacteria

Mycoplasma pneumoniae

• Unique Biology: The smallest free-living, self-replicating organisms ($0.2\text{--}0.3\ \mu\text{m}$). They completely lack a rigid peptidoglycan cell wall, enclosed solely by a fragile trilaminar cell membrane containing sterols scavenged from host tissues. Because they lack a peptidoglycan cell wall, they are intrinsically resistant to all $\beta$-lactam antibiotics (penicillins, cephalosporins, carbapenems) and are invisible on Gram stains.
• Cultivation: Inoculated onto enriched PPLO / Hayflick agar supplemented with horse serum (source of cholesterol) and yeast extract. Microscopic inspection under low power reveals tiny colonies with a dense central core growing down into the agar, presenting a classic "fried-egg" colonial morphology.
• Clinical Presentation: Primary etiology of primary atypical pneumonia ("walking pneumonia") in children and young adults.
• Cold Agglutinins: Elicits the transient production of cold agglutinins in over 50% of patients—autoantibodies of the IgM class directed against the erythrocyte I antigen. In the laboratory, cold agglutinins agglutinate human adult Group O erythrocytes at $4^\circ\text{C}$, but disperse completely upon warming to $37^\circ\text{C}$.

Chlamydia trachomatis

• Obligate Intracellular Parasitism: Possesses an inner and outer membrane resembling Gram-negative bacteria, but lacks detectable muramic acid. Dependent on the host cell for high-energy ATP ("energy parasites").
• Biphasic Developmental Cycle:
  1. Elementary Body (EB): Small ($0.3\ \mu\text{m}$), rigid, heavily disulfide-crosslinked, metabolically inert, environmentally stable extracellular unit; the infectious form that binds and enters host epithelial cells.
  2. Reticulate Body (RB): Larger ($1.0\ \mu\text{m}$), non-crosslinked, metabolically active intracellular unit; the replicative form that divides by binary fission within a membrane-bound cytoplasmic vacuole (inclusion body). After 48 to 72 hours, RBs condense back into EBs, lysing the host cell to repeat the cycle.

                         Chlamydia Developmental Cycle
      [ Extracellular ]  Elementary Body (EB) ──► Binds Host Epithelial Cell
                                                        │
                                                        ▼ Endocytosis
      [ Intracellular ]  Reorganizes into Reticulate Body (RB) within Vacuole
                                                        │
                                                        ▼ Binary Fission (24-48h)
                         Multiple RBs form Cytoplasmic Inclusion Body
                                                        │
                                                        ▼ Condensation
                         RBs condense back into Elementary Bodies (EBs)
                                                        │
      [ Extracellular ]  Host Cell Lysis ──► Infectious EBs released to infect

• Serovars and Diseases:
  • Serovars A, B, Ba, C: Endemic blinding trachoma (chronic follicular keratoconjunctivitis, pannus formation, trichiasis, and blindness).
  • Serovars D through K: Sexually transmitted infections: non-gonococcal urethritis (NGU), cervicitis, pelvic inflammatory disease (PID, ectopic pregnancy, tubal factor infertility), and neonatal inclusion conjunctivitis.
  • Serovars L1, L2, L3: Lymphogranuloma venereum (LGV): invasive STD with transient genital ulcers followed by painful suppurative inguinal buboes separated by the inguinal ligament ("groove sign").
• Laboratory Diagnosis: NAAT is the clinical standard. Direct cytology of conjunctival scrapings reveals basophilic intracytoplasmic inclusion bodies that stain brown with Lugol's iodine (due to glycogen matrix).

Rickettsia and Orientia (The Weil-Felix Agglutination Reaction)

Obligate intracellular Gram-negative bacteria transmitted by arthropod vectors (ticks, fleas, lice, and mites).

• Rickettsia prowazekii: Epidemic louse-borne typhus (Pediculus humanus vector).
• Rickettsia rickettsii: Rocky Mountain spotted fever (Dermacentor tick vector).
• Orientia tsutsugamushi: Scrub typhus, endemic throughout the Asia-Pacific region, transmitted by larval trombiculid chigger mites (Leptotrombidium).
• The Weil-Felix Reaction: Historic presumptive serological test based on sharing of cross-reacting polysaccharide somatic O antigens between specific Rickettsia strains and non-motile strains of $\textit{Proteus}$:
  • Proteus OX-19 & OX-2: Agglutinated strongly by serum from patients with epidemic/endemic typhus and spotted fevers.
  • Proteus OX-K: Agglutinated strongly by serum from patients with scrub typhus (Orientia tsutsugamushi), while OX-19 and OX-2 remain negative.

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5. Public Health and Environmental Microbiology

Bacteriological Examination of Drinking Water

Standard water surveillance programs rely on detecting indicator organisms rather than isolating scarce, episodic enteric pathogens directly.

Criteria for Coliform Indicator Organisms

• Total Coliforms: Aerobic and facultatively anaerobic, Gram-negative, non-spore-forming bacilli that ferment lactose with gas and acid production within 48 hours at $35^\circ\text{C}$. Members include Escherichia, Klebsiella, Enterobacter, and Citrobacter.
• Fecal (Thermotolerant) Coliforms: Coliforms that ferment lactose with gas production within 24 hours at an elevated incubation temperature of $44.5 \pm 0.2^\circ\text{C}$. Primarily composed of Escherichia coli; confirms direct, recent fecal contamination by warm-blooded animals.

The Multiple Tube Fermentation (MTF) Technique

The MTF assay calculates the bacterial density using a statistical index termed the Most Probable Number (MPN) per 100 mL of water, executed in three sequential phases:

                       Multiple Tube Fermentation (MTF) Stages
                                           │
                   ┌───────────────────────┼───────────────────────┐
                   ▼                       ▼                       ▼
            Presumptive Phase        Confirmed Phase        Completed Phase
         • Lauryl Tryptose Broth  • BGLB Broth at 35°C    • Eosin Methylene Blue
           (LTB) with inverted      (Total Coliforms)       (EMB) Agar
           Durham tube            • EC Broth at 44.5°C    • Nucleated colonies
         • Incubate 24-48h at 35°C  (Fecal Coliforms)       with metallic sheen
         • Turbidity + Gas = +    • Gas in Durham = +     • Gram smear: Gram-neg rods
                                                          • Lactose broth gas = confirmed

1. Presumptive Phase: Replicate tubes of Lauryl Tryptose Broth (LTB) (containing sodium lauryl sulfate to inhibit non-coliforms) containing inverted Durham fermentation tubes are inoculated with water sample aliquots ($10\text{ mL}, 1.0\text{ mL}, 0.1\text{ mL}$). Incubated at $35^\circ\text{C}$ for $24 \pm 2\text{ hours}$ (extended to 48 hours if negative). The presence of turbidity and gas formation in the Durham tube constitutes a positive presumptive test.
2. Confirmed Phase: Inoculum from every positive LTB tube is transferred using a sterile loop into:
   • Brilliant Green Lactose Bile (BGLB) Broth: Contains bile salts and brilliant green dye to inhibit non-coliform spore-formers. Incubated at $35^\circ\text{C}$ for 48 hours. Gas production confirms Total Coliforms.
   • EC Broth: Inoculated tubes are immersed in a circulating thermoregulated water bath maintained at $44.5 \pm 0.2^\circ\text{C}$ for $24 \pm 2\text{ hours}$. Gas production confirms Fecal Coliforms.
3. Completed Phase: Positive confirmed tubes are streaked onto Eosin Methylene Blue (EMB) agar. Typical nucleated colonies with or without a greenish-metallic sheen are subcultured into nutrient agar slants and lactose fermentation broths. Production of gas in lactose broth and demonstration of Gram-negative non-spore-forming bacilli on microscopic smears completes the test.

• MPN Index Calculation: The combination of positive and negative tubes across the three dilutions (e.g., 5-3-1) is cross-referenced against standardized Hoskins / de Man MPN statistical probability tables to derive the MPN of coliforms per 100 mL of water.

Membrane Filtration (MF) Method

A field and laboratory standard method. Exactly $100\text{ mL}$ of potable water is passed through a sterile cellulose ester membrane filter with a standardized pore size of $0.45\ \mu\text{m}$, which traps all bacterial cells.

• The membrane is placed onto a pad saturated with m-Endo medium and incubated at $35^\circ\text{C}$ for 24 hours. Coliforms ferment lactose, releasing acetaldehyde that reacts with sodium sulfite and basic fuchsin to produce red colonies displaying a striking golden-green metallic sheen.
• For fecal coliforms, the membrane is placed on m-FC medium and incubated at $44.5^\circ\text{C}$, producing distinct blue colonies.
• Potable Water Standard: The Philippine National Standards for Drinking Water of 2017 (DOH Administrative Order No. 2017-0010) require total coliforms of $< 1.1\text{ MPN}/100\text{ mL}$ by multiple tube fermentation (or $< 1$ colony per $100\text{ mL}$ by membrane filtration), thermotolerant coliforms or E. coli likewise below detection, and a heterotrophic plate count $< 500\text{ CFU/mL}$.

Microbiological Quality Control of Dairy Products

• Standard Plate Count (SPC): Assesses the viable aerobic bacterial load of milk. Serial decimal dilutions of milk are mixed with Plate Count Agar using the pour-plate method and incubated at $32^\circ\text{C}$ for $48 \pm 3\text{ hours}$. Grade A raw milk from an individual producer must not exceed $100,000\text{ CFU/mL}$; after commingling, it must not exceed $300,000\text{ CFU/mL}$. Grade A pasteurized milk must not exceed $20,000\text{ CFU/mL}$ with a coliform count $\le 10\text{ CFU/mL}$.
• The Phosphatase Test (Alkaline Phosphatase / ALP): The gold-standard regulatory chemical assay for verifying adequate pasteurization. Bovine milk contains native alkaline phosphatase, an enzyme whose thermal denaturation curve is slightly higher than that of the most heat-resistant non-spore-forming pathogen found in raw milk—Coxiella burnetii (the rickettsial agent of Q fever). When milk undergoes proper pasteurization (High-Temperature Short-Time HTST: $72^\circ\text{C}$ for 15 seconds), alkaline phosphatase is completely destroyed. Detection of residual alkaline phosphatase activity indicates underpasteurization or post-pasteurization contamination with raw milk.
• Food Contact Surface Swab Sanitation: The swab-rinse technique uses a sterile Dacron swab moistened in buffered rinse solution to sample a standardized area of food service utensils ($100\text{ cm}^2$ or rims of glasses). Acceptable sanitary standards dictate a plate count of $< 100\text{ colonies per utensil}$ or $< 1\text{ CFU/cm}^2$ with a complete absence of coliforms.