Production-scale inactivated Mycoplasma gallisepticum bacterin intended for parenteral administration is formulated most commonly as a water-in-oil emulsion. The antigen concentrate is prepared from a broth culture that has been inactivated with formaldehyde at 0.1–0.2% v/v for 18–24 h at 37°C and then clarified by depth filtration. The aqueous phase is adjusted to a protein nitrogen content that corresponds to the manufacturer's target antigen mass per dose, typically verified by a species-specific sandwich ELISA or haemagglutination inhibition assay. This aqueous phase is combined with a light mineral oil phase containing sorbitan monooleate and polysorbate 80 at a ratio of 70:30 aqueous-to-oil, although a phase ratio between 0.30 and 0.35 non-aqueous volume fraction may be used when stability data support it. Emulsification is performed in a jacketed stainless steel vessel fitted with a Silverson type rotor-stator mixer operating at 3500–5000 rpm for 10–20 min; droplet size distribution is measured by laser diffraction following ISO 13320-1:2020, with a Dv90 below 2.0 µm and a Dv10 above 0.4 µm used as practical release targets. Excessive interface area arising from a very small Dv10 can promote antigen denaturation, while a Dv90 above 2.0 µm is associated with accelerated creaming and potential dose non-uniformity during filling. The high-shear step generates a measurable temperature rise; the batch is therefore passed through a plate heat exchanger set to 18–22°C, and excursions above 45°C for more than 5 min are treated as a deviation because mycoplasma membrane proteins aggregate under thermal stress. Filling is conducted under Grade A laminar airflow with a peristaltic filling line operating at 60–120 units/min, and the headspace is gassed with nitrogen to reduce oxidative degradation of the oil phase. Release testing includes sterility per Ph. Eur. 2.6.1, bacterial endotoxin per Ph. Eur. 2.6.14, viscosity per ISO 2555 at 25°C, and droplet size stability after 24 h storage at 4°C.

What Happens to Antigen Conformation During Aluminium Hydroxide Adsorption?

Aluminium hydroxide gel is an alternative adjuvant system for aqueous injectable suspensions of inactivated Mycoplasma gallisepticum antigen, particularly when oil-based adjuvants are not desirable for layer pullets. Adsorption is carried out at pH 6.0–7.0, because membrane-associated antigen proteins carry a net negative charge in this range and bind electrostatically to the positively charged gel surface. The antigen dose is expressed in haemagglutination inhibition titre units per dose rather than colony-forming units, because the inactivation process destroys viability and only serological identity remains meaningful. A typical batch is prepared by adding 1.0–2.0% w/v aluminium hydroxide gel to the antigen concentrate under low-shear stainless steel agitation at 4–8°C for 4–8 h. The binding capacity must be confirmed by measuring free protein in the supernatant after 24 h using a Bradford or bicinchoninic acid assay; a free protein value above 10% of total added protein indicates gel overloading and requires a reduction in antigen loading or an increase in gel concentration. Phosphate buffers above 10 mM are incompatible with the aluminium gel because phosphate displaces hydroxide groups and changes the gel surface charge, reducing antigen adsorption efficiency. The final aqueous suspension is filled into Type II glass vials under aseptic conditions; addition of 0.5–1.0% sodium carboxymethylcellulose or a carbomer improves suspension uniformity during storage but must be validated for compatibility with the antigen and the aluminium adjuvant. The settled bed volume after 7 days at 4°C is recorded as a simple physical stability indicator; a bed volume below 80% of total volume after resuspension may indicate irreversible aggregation and poor dose uniformity in the field.

Lyophilised bulk antigen is the preferred physical state for contract manufacturing operations that ship inactivated Mycoplasma gallisepticum antigen between sterile production sites or store it for later formulation into solid dosage forms. Freeze-drying requires a stabiliser blend composed of 2.0–5.0% w/v sucrose, 1.0% w/v mannitol, and 0.5% w/v hydrolysed gelatin, buffered to pH 7.0–7.4. The collapse temperature of the formulated solution is determined by freeze-drying microscopy before cycle design; if the collapse temperature is below -20°C, primary drying shelf temperature is set at -25°C to -15°C and chamber pressure at 50–100 µbar for 24–48 h. Secondary drying at 20–30°C for 4–8 h reduces residual moisture to 1.0–2.0%, measured by coulometric Karl Fischer titration according to Ph. Eur. 2.5.32. The dried cake is milled through a conical mill fitted with a 0.5 mm screen under nitrogen to produce a free-flowing powder; colloidal silicon dioxide at 0.2–0.5% is added as a glidant and the powder flow characterised by Ph. Eur. 2.9.36. A Carr index below 25 is needed for consistent downstream filling. The produced powder is then split into separate campaigns for tablet, capsule, or premix manufacturing, each requiring its own validation because the antigen particle size and moisture content affect blend segregation and dissolution behaviour differently.

When Tablet Compaction Requires Dry Granulation Because Antigen Glass Transition Starts Below Compression Force

Direct compression of a lyophilised Mycoplasma gallisepticum antigen blend becomes unreliable when tablet hardness above 50 N is specified on a rotary tablet press. The antigen-containing powder has a low glass transition temperature because the stabiliser sucrose at 2.0–5.0% w/v absorbs ambient moisture and plasticises the matrix. Roller compaction is therefore introduced at a roll pressure of 3–5 kN/cm, a roll speed of 3–7 rpm, and a gap of 0.8–1.2 mm, followed by dry milling through a granulator with a 1.0 mm screen. The resulting granules are blended with crospovidone at 2.0–5.0% as disintegrant, sodium stearyl fumarate at 0.5–1.0% as lubricant, and microcrystalline cellulose as filler. Tablet compression is performed on a rotary press equipped with a compression force monitor; the main compression force is capped at 12–18 kN and dwell time at 10–30 ms to keep the die wall temperature below 40°C. Tablet hardness is measured by Ph. Eur. 2.9.8 and friability by Ph. Eur. 2.9.7; a friability below 1% is required for film coating. Capsule filling with the same granules is performed on a dosator or tamping pin machine at 60–100 capsules/min; capsule shells must be low-moisture hypromellose to avoid water uptake by the granulate. The tablet core is not enteric-coated unless the intended delivery route is oral mucosal challenge; when enteric coating is applied, a hydroxypropyl methylcellulose acetate succinate film at 5–8% weight gain is used, and the coated tablets are tested for disintegration per Ph. Eur. 2.9.1 and for antigen potency after dissolution. Because the antigen is inactivated, crushing or chewing of tablets prior to ingestion does not simulate a viable vaccine response, and any oral tablet claim must be validated in a specific challenge model rather than extrapolated from parenteral potency data. Published data for oral bioavailability of inactivated Mycoplasma gallisepticum antigen in poultry is limited.

Dry feed premix preparation of inactivated Mycoplasma gallisepticum antigen is limited to experimental oral inoculation studies and is not a registered veterinary vaccine indication. The lyophilised powder is blended with lactose monohydrate or wheat middlings in a double-cone blender operating at 10–15 rpm for 20 min. Antigen content per gram is typically 500–2000 haemagglutination inhibition units, which makes segregation during bin discharge a critical process failure. Blend uniformity is assessed by sampling 10–20 points across the batch according to Ph. Eur. 2.9.40; the coefficient of variation must remain below 5.0% before use. Moisture uptake is controlled because the powder adsorbs water at relative humidity above 60% within 2–4 h, forming agglomerates that can cause non-uniform distribution in complete feed. The premix is packed in polyethylene-lined aluminium foil bags with a desiccant sachet and stored at 2–8°C. At the feed mill, the premix is diluted into complete poultry rations at 1:100 or 1:200 using a horizontal ribbon mixer for 15 min; the mixer temperature must remain below 80°C because the antigen denatures at pelleting and extrusion temperatures. Steam conditioning at 80–90°C is incompatible with this inactivated antigen, and the product is therefore administered as a top-dress rather than incorporated into pelleted feed. Segregation after dilution is monitored by testing 5–10 feed samples for antigen content; a relative standard deviation above 10% triggers re-blending or particle size adjustment of the carrier.

Residual Formaldehyde, Thiomersal, and Preservative Compatibility in Multi-Dose Injectable Presentations

Multi-dose vials of inactivated Mycoplasma gallisepticum vaccine require an antimicrobial preservative when multiple broiler breeder birds are vaccinated from a single rubber-stoppered vial. Formaldehyde used for antigen inactivation is neutralised with sodium bisulphite to a residual level below 0.02% w/v before final formulation; residual formaldehyde can react with thiomersal and reduce its preservative activity. If thiomersal is included at 0.01% w/v, the finished vial must pass preservative effectiveness testing according to Ph. Eur. 5.1.3, with at least a 3 log reduction in Pseudomonas aeruginosa and Staphylococcus aureus counts within 7 days. Phenoxyethanol at 0.5% w/v is an alternative preservative that is less reactive with residual formaldehyde, but it can increase emulsion viscosity and alter droplet interface tension. In aqueous aluminium hydroxide suspensions, thiomersal binds to the gel surface, and the free preservative concentration must be verified by HPLC after 24 h; a free level below 80% of the added amount is a known stability risk. Rubber stoppers for multi-dose vials must be chlorobutyl-coated to limit leachable adsorption of antigen; silicone oil applied to stopper surfaces can strip antigen from solution during repeated broaching. The in-broach potency after 10 simulated needle insertions is evaluated by haemagglutination inhibition assay to confirm that the antigen titre remains within the label claim.

Drinking water solutions of inactivated Mycoplasma gallisepticum antigen are not commercial veterinary vaccines; they appear only in mucosal immunogenicity studies in specific-pathogen-free layer chicks. The freeze-dried or liquid antigen is diluted into deionised water or municipal water that has been treated with 0.1% w/v sodium thiosulfate to neutralise free chlorine. The final solution is adjusted to pH 6.8–7.2 and osmolality 280–320 mOsm/kg; addition of 0.5% w/v bovine serum albumin or 1.0% w/v casein hydrolysate reduces surface adsorption of antigen to plastic drinker lines. Because the antigen is not replication-competent, mucosal delivery alone normally produces weak serum antibody titres; adjuvanted oral formulations or enteric-coated presentations are needed for any meaningful intestinal uptake. The prepared solution must be consumed within 4 h at 25°C; holding beyond 6 h results in a loss of haemagglutination inhibition activity of more than 0.5 log₂ titre. Residual chlorine must be below 0.1 mg/L at the point of administration, and drinker lines must be flushed with thiosulfate-treated water before dosing to remove biofilm and metal ion residues that can bind antigen.