Live Brucella suis S2 antigen intended for injectable freeze-dried veterinary vaccines is normally produced from a seed lot system in which the working seed is propagated on Brucella agar or in tryptic soy broth supplemented with serum under controlled CO₂ tension. The harvested broth is concentrated by tangential-flow filtration or disc-stack centrifugation, washed with chilled phosphate-buffered saline, and suspended in a stabiliser vehicle before aseptic filling into depyrogenated Type I borosilicate vials. Aseptic processing is conducted under EU GMP Annex 2 for biological active substances and ISO 14644-1:2015 class A conditions within a class B background. Viable count detection on Brucella agar after 72–96 h at 37 °C in 5–10% CO₂ is used to track batch titre. Lyophilisation cycle parameters are critical: shelf cooling to -40 °C before primary drying, chamber pressure maintained at 10–20 Pa, and secondary drying at +20 °C to +25 °C until residual moisture by Karl Fischer titration is below 3.0% w/w. The collapse temperature of S2-containing cakes containing sucrose and gelatin must be determined by freeze-drying microscopy for each lot because excipient lot-to-lot variation shifts the glass transition. Filling line speed, stopper residual moisture, and vial breakage during lyophilisation are monitored because non-uniform cake shrinkage at the vial wall increases the risk of elevated moisture pockets that reduce viable count stability. For injectable use, reconstitution with sterile diluent should produce a uniform suspension within 60 s without large flocculates; particle size after reconstitution is measured by laser diffraction, with D50 typically below 30 μm if the vaccine is to pass through a 23 G needle. The preparation is not indicated for pregnant animals, and accidental self-injection requires immediate medical attention due to the zoonotic potential of live Brucella. Real-time stability of the freeze-dried injectable is preferred over accelerated thermal screening because live bacterial death kinetics in dried matrices frequently deviate from Arrhenius predictions, and vial-to-vial moisture variability below 1.0% w/w can produce non-linear viable count loss.

What Limits Direct Compression of Live S2 Lyophilizate into Oral Tablet Matrices?

Direct compression of live Brucella S2 lyophilizate into oral tablets for mass oral vaccination of sheep and goats is constrained by the thermo- and moisture sensitivity of the bacterium. The raw dried API is milled under chilled nitrogen or in a low-humidity isolator to avoid frictional heating above 25 °C, because viability loss accelerates when the powder bed exceeds 30 °C for more than a few minutes. Excipients are selected for low equilibrium moisture and acid-neutralising capacity: anhydrous lactose, microcrystalline cellulose with water activity below 0.3, sodium bicarbonate at 2–4% w/w, and sodium citrate dihydrate as a buffering pair. A V-blender charged to 50–65% of working volume and rotated at 8–12 rpm for 15–20 min yields acceptable blend uniformity, but over-blending causes segregation of the low-density lyophilised API. Lubrication with magnesium stearate must not exceed 0.5% w/w, and mixing time with lubricant should remain below 3 min because hydrophobic films delay tablet disintegration and reduce the recovery of viable S2 in simulated rumen fluid. Compression on a rotary tablet press is operated at 8–16 kN to produce tablets with hardness of 60–100 N and friability below 1.0% when tested according to USP <1216>. Enteric coating is necessary for oral dosing because unprotected tablets release in the abomasum or stomach at pH 2.0–4.0, where live S2 viability falls rapidly. Aqueous enteric coating with poly(methacrylic acid-co-ethyl acrylate) dispersion is applied in a side-vented coating pan at inlet air 35–40 °C, product bed temperature 25–30 °C, and spray rate not exceeding 6 g/min per kg of tablet bed; higher spray rates cause film tackiness and over-wetting that can reduce CFU recovery. The coated tablets are tested using USP <711> delayed-release method: 2 h in 0.1 N HCl at 37 °C shows no visible disintegration and not more than 10% release, followed by phosphate buffer pH 6.8 with complete release within 45 min. Because each formulation lot may differ in excipient moisture, the acid-neutralising capacity should be verified by titration to pH 5.5; published buffer capacity data for S2-specific oral tablet matrices are limited, requiring development-scale wet-lab confirmation. Organic-solvent-based coating should be avoided because residual ethanol or acetone in the film and granule can denature outer membrane proteins of live Brucella, reducing colonisation competence.

In countries where oral S2 vaccination is delivered through compound feed for small ruminants, the live API is handled as a high-risk biological material and not as a conventional chemical powder. Premixes for medicated feed containing live Brucella S2 are prepared by geometric dilution of the lyophilised API into a lactose monohydrate or dextrose carrier at ambient relative humidity below 30% RH. Ribbon blenders with slow rotor speed of 20–30 rpm and plough mixers operating without high-shear choppers are used because high tip speeds create frictional heat above 30 °C and reduce viable count. Homogeneity acceptance follows the same blend uniformity logic as conventional premixes, but the active is enumerated as CFU rather than by HPLC; sampling across 10–20 points should yield a coefficient of variation below 5.0%. Carrier particle size D50 between 150–300 μm matches the API powder density to reduce segregation during transfer. Water activity of the finished premix is maintained below 0.4, measured by dew-point hygrometry or a chilled-mirror instrument, because higher water activity increases membrane lipid oxidation and accelerates death of the live strain during storage at 2–8 °C. The premix is not incorporated before pellet conditioning at 70–85 °C; thermal death of live S2 is rapid at those temperatures, so the product is applied post-pellet by liquid spray or micro-dosing into cool pellets or meal. In feed formulations, the concurrent presence of tetracycline or amphenicol medicated premixes is incompatible with live Brucella S2 replication and must be segregated in production scheduling and feeding calendars. Label recommendations under official veterinary GMP and Chinese Veterinary Pharmacopoeia requirements define a final viable count per gram in feed; published data for specific S2 formulations across different feed matrices are limited, and field verification of CFU recovery from feed is required. Feed mill decontamination after handling live Brucella S2 uses alkaline detergents and not steam alone, because viable cells may survive in dry dust on equipment surfaces. Packaging lines should run at below 25 °C and below 40% RH, and aluminium laminate pouches with oxygen barrier and desiccant are used rather than woven paper bags that permit moisture ingress.

Hard Gelatin Capsule Filling and Enteric Polymer Disintegration in Live Brucella S2 Dosage Forms

Oral capsules containing live Brucella S2 are typically limited to small-scale clinical or targeted herd vaccination rather than mass feed application because the capsule shell itself introduces a moisture reservoir. Powder filling is performed on a dosator or tamping-pin encapsulator at 20–30% RH and 18–22 °C, because brittleness increases below 40% relative humidity for hard gelatin, while moisture above 50% RH transfers from the capsule shell into the dry powder fill and depresses viable count. Hydroxypropyl methylcellulose capsules are preferred where the fill contains hygroscopic sodium bicarbonate, because gelatin cross-linking with aldehydes from oxidised excipients retards dissolution. Fill weight for a size 0 capsule is commonly 300–450 mg, composed of the S2 lyophilizate, anhydrous lactose, microcrystalline cellulose, and a 0.2–0.5% w/w hydrophobic lubricant; the API is hand-screened or passed through a 500 μm sieve before blending to break soft aggregates without generating heat. Dissolution testing uses USP <711> with a two-stage acid-to-neutral buffer protocol; capsule disintegration is evaluated under USP <701> with disintegration fluid at 37 °C. Enteric protection for capsules is more difficult than for tablets because film adhesion to the capsule cap and body junction is incomplete; a subcoating of hydroxypropyl methylcellulose at 2–3% weight gain is applied before the enteric polymer. The enteric-coated capsules must resist 0.1 N HCl for 2 h without leaking; leakage is detected by visual inspection of the medium or by dye ingress test. After transfer to phosphate buffer pH 6.8, release should occur within 60 min; slower release may expose S2 to neutral pH conditions that support growth but also to pancreatic enzymes in the intestinal lumen, and viability loss after dissolution is frequently higher than predicted by in vitro aqueous buffers. The operational boundary is strict: capsules should not be stored above 25 °C for longer than 24 h unless stability data specific to the HPMC capsule formulation are generated. Packaging in Alu-Alu blister with desiccant is required for tropical climates because capsule shells undergo moisture-dependent glass transition and distortion that can create breaches in the enteric coating.

When the Lyophilised Powder Is Reconstituted for Oral Drench or Drinking Water Administration

Reconstitution of live S2 lyophilizate for oral drench or drinking water vaccination shifts the stability risk from desiccation to aqueous thermal inactivation. The diluent is typically sterile phosphate-buffered saline at pH 7.2 or chilled physiological saline; calcium- or magnesium-containing hard water should be avoided because divalent cations destabilise bacterial outer membranes and increase adsorption to the sides of the drinking vessel. In drench administration, the reconstituted suspension is transferred to a dosing gun calibrated to deliver 2–5 mL per animal; the suspension should be agitated every 5 min because the live cells settle rapidly, causing dose-to-dose variability that can exceed 20% CV in field use. For drinking water, chlorine residuals above 0.1 mg/L must be neutralised with sodium thiosulfate at 0.2–0.5 g/L, and organic matter in dirty troughs reduces recoverable CFU within 30 min. Water temperature should remain below 15 °C during the vaccination window; at 25 °C the viable count may decline by 1 log10 within 2–4 h depending on the matrix, but published data for S2 in specific field water sources are limited. The product is used within 2 h after reconstitution according to standard veterinary vaccine handling guidance, though this limit is conservative and derived from general live bacterial vaccine stability, not from a universal S2-specific kinetic curve. No antimicrobial residues should be present in the water line, and quaternary ammonium disinfectants used in house cleaning must be rinsed before vaccination because even sub-lethal concentrations stress the strain and reduce mucosal colonisation. Solutions are not terminally sterilised after reconstitution; therefore, all equipment is depyrogenated and rinsed with sterile diluent. Operators must wear full face protection during drenching because accidental splashing of live Brucella S2 suspension onto mucous membranes is a recognised occupational exposure route.

Granulation of a live Brucella S2 dry blend imposes stricter thermal limits than conventional wet massing of chemical APIs because the binder spray, drying air, and frictional forces in the granulator must be balanced against bacterial death kinetics. Fluid-bed top-spray granulation with an aqueous binder containing 2–3% w/w polyvinylpyrrolidone K30 and 1–2% w/w trehalose is operated at inlet air temperature 30–35 °C, product bed temperature 24–26 °C, and atomising air pressure of 1.0–1.5 bar; higher product temperatures generate granules with acceptable flow but reduce CFU recovery by more than the limit of detection in subsequent plate counts. Spray rate is staged and not allowed to exceed 10 g/min per kg of dry powder because wet massing creates localised high-moisture zones where the live cells undergo osmotic shock and premature metabolic activation. After granulation, drying continues until outlet air relative humidity falls below 20% RH and granule water activity is below 0.4. The dried granules are screened through a 1.0 mm conical mill at low speed to produce a target particle-size distribution with D50 180–350 μm and not more than 15% fines below 100 μm, since fines segregate in subsequent premix packaging and create dosing variance. Granulated S2 intermediates are filled into double polyethylene bags inside aluminium laminate pouches with desiccant; residual moisture by Karl Fischer titration should remain below 3.0% w/w at release and below 4.0% w/w at end of shelf life under 2–8 °C storage. Batch-to-batch variance in viable count after granulation frequently exceeds 0.5 log10 when the API lyophilizate cake has collapsed or contains high residual moisture; therefore, incoming API water activity and cake appearance are critical release parameters before granulation. The finished granules are dispersed in drinking water or drench fluid at the point of administration, and the dispersion should be used within 2 h. Granulation is not appropriate for all S2 presentations; some manufacturers avoid it entirely for injectable routes because the additional thermal exposure reduces the viable count and increases the risk of failing the release specification. Clean room gowning and mask use are mandatory during granule transfer because fine dried powder containing live Brucella can become airborne and contaminate non-target surfaces.