Salmonella abortus-equi Vaccine, Live (Strain C355) Veterinary Grade API is a bulk-processed live attenuated bacterial active pharmaceutical ingredient derived from Salmonella enterica subsp. enterica serovar Abortusequi isolate C355. It is supplied as a lyophilized cake, lyophilized powder, or frozen concentrated suspension for downstream manufacture of tablets, injectable suspensions, capsules, powders, granules, premixes, and solutions. The material is intended solely for licensed veterinary pharmaceutical manufacturing; the API is not a finished dosage form and is not intended for direct administration. Fermentation is performed in stirred-tank bioreactors at 37 °C with dissolved oxygen controlled at 30% of air saturation and pH maintained at 7.0 ± 0.2. After concentration by tangential-flow filtration and lyophilization in the presence of cryoprotectants such as sucrose and skim milk, the bulk API is milled under cryogenic conditions to a particle size suitable for dry blending. The critical quality attributes are viable cell count, serological identity, residual moisture, water activity, reconstitution pH, and absence of specified extraneous microorganisms. Viable count is determined by spread-plate or membrane-filtration assay on tryptic soy agar incubated at 37 ± 2 °C for 24–48 h under aerobic conditions and is expressed as CFU/g or CFU/mL. Identity is confirmed by slide agglutination with O4, O12 antiserum and by polymerase chain reaction amplification of a strain-specific marker. Lyophilized lots are controlled to residual moisture ≤ 3.0% w/w and water activity ≤ 0.30 to reduce metabolic injury during storage at 2–8 °C; frozen suspension lots are stored at −70 °C with a lower acceptance limit of −60 °C.

What Limits Viable Count Recovery in Tablet and Capsule Manufacture?

Tableting of live C355 biomass is constrained by compaction pressure, dwell time, and feed blend moisture. Direct compression is preferred over wet granulation because aqueous granulation can activate residual metabolic pathways and produce viability losses greater than 1.0 log₁₀ CFU/g during drying. Formulations based on mannitol or microcrystalline cellulose with particle-size distributions between 50 µm and 250 µm allow uniform die filling at low lubricant content. Compression pressure is maintained below 120 MPa, turret speed below 25 rpm, and room relative humidity below 20%. Under these conditions, viability loss is usually limited to 0.3–0.8 log₁₀ CFU/g; above 150 MPa, adiabatic temperature rise and shear cause additional losses exceeding 1.0 log₁₀ CFU/g. Tablet hardness is maintained at 30–50 N to preserve rapid disintegration, and friability is controlled to ≤ 1.0% according to USP <1216>. Sustained-release coatings are avoided because prolonged gastric residence at pH 2.0–4.0 reduces viability by more than 2.0 log₁₀ CFU/g in enteric Gram-negative bacteria. Published data for this specific C355 configuration is limited; therefore each compression run requires post-compression viable count and moisture audit. Encapsulation is performed in hard gelatin or hypromellose capsules at 25 °C ± 2 °C and ≤ 20% RH. Low-speed capsule filling equipment is used because high-speed dosator filling can generate frictional heat at the die pin and reduce viability.

When Reconstituted Parenteral Suspensions Must Meet Cold-Chain and Sterility-Assurance Boundaries

For injectable manufacture, the frozen concentrate or lyophilized cake is reconstituted under aseptic conditions in sterile isotonic diluent. The finished suspension is controlled for pH 6.5–7.5 and osmolality 280–320 mOsm/kg. Sterility testing is performed according to Ph. Eur. 2.6.1 with appropriate neutralization because the intentional live vaccine strain grows on standard media; the test is interpreted as absence of specified extraneous microorganisms rather than absolute sterility. Mycoplasma absence is verified by Ph. Eur. 2.6.7, and bacterial endotoxin is controlled by Ph. Eur. 2.6.14 when the final route is parenteral. The diluent is used at 15–25 °C during reconstitution; chilled diluents below 4 °C can increase osmotic membrane stress on the bacterial cells. The cold chain from bulk collection to filling maintains 2–8 °C for short-term liquid handling and −70 °C for frozen storage. Any hold time above 8 °C is limited to 4 h to avoid logarithmic loss of viable count. Final injectable suspensions are filled into multi-dose glass vials with headspace purged with nitrogen to minimize oxidative stress on the bacterial cells.

Processing into oral powders, granules, and premixes is governed by particle segregation and water activity rather than by injectable sterility. The lyophilized powder is mixed with a diluent carrier, typically spray-dried lactose or maltodextrin, to an active concentration of 10⁹–10¹¹ CFU/g depending on the final dose target. The API is sieved through 250 µm mesh after milling to match carrier particle size and reduce segregation. Mixing is performed in a low-shear double-cone or V-blender at 20–25 °C and ≤ 20% RH; the target blend uniformity is ≤ 5.0% coefficient of variation for viable count in ten samples taken from the blender discharge. Mixing time is optimized by sampling at 5 min, 10 min, and 15 min; over-mixing beyond 20 min can increase static charge and segregation. Granulation, if required, is carried out by dry roller compaction at pressures below 40 MPa or by fluid-bed spray granulation with inlet air at 35 °C maximum and product temperature not exceeding 28 °C. Premix products are filled into foil-lined pouches with desiccant to maintain water activity below 0.30 during the assigned shelf life.

Attenuation Profile, Target Species Usage, and Cross-Protective Boundaries

The API is used for active immunization of horses against Salmonella abortus-equi, the historical causal agent of equine paratyphoid and late-gestation abortion in mares. Strain C355 carries attenuation markers that reduce invasiveness while retaining limited transient replication in intestinal and lymphoid tissues. The intended immunological outcome is induction of mucosal IgA and systemic IgG without causing the abortion associated with wild-type infection. The route of administration of the final product may be intramuscular, subcutaneous, or oral depending on the registered formulation; oral powders, granules, and premixes are generally mixed into feed immediately before use. Published field regimens often include two doses administered 3–4 weeks apart, followed by an annual booster, but final dosage regimens are registration-specific. The live API does not confer defined cross-protection against Salmonella enterica serovars Typhimurium or Enteritidis unless challenge studies demonstrate otherwise. Published challenge data for C355-specific cross-serovar protection is limited, and field usage should not extrapolate across serovars.

Regulatory and Manufacturing Profiles Distinguish This Attenuated Strain from Autogenous Bacterins

Compared with inactivated bacterins, the live C355 API requires cold-chain management, viability monitoring, and stricter processing controls. Inactivated products are non-replicating and may be stored with less stringent temperature limits; however, they typically require oil or aluminum hydroxide adjuvants to achieve satisfactory antibody responses. The live C355 vaccine usually requires no adjuvant and can induce cell-mediated and mucosal immune responses that inactivated products may not stimulate to the same degree. The difference is operationally relevant in downstream manufacturing: lyophilization and dry blending replace sterile adjuvanted-emulsion processing, and batch release must include viable count rather than only antigen content. Cold-chain failure is a defined batch rejection criterion for live C355; a temperature excursion above 25 °C for more than 2 h after lyophilization triggers viability retesting against release specifications. Inactivated bacterins do not carry this same rejection parameter. Real-time stability samples for the live API are retained at 2–8 °C and −70 °C to support the assigned shelf life.

Liquid oral solutions and drench formulations are prepared only at the point of final packaging because aqueous shelf life of live C355 is limited. Stability in aqueous vehicles at 2–8 °C is acceptable for 24–48 h; formulations buffered with phosphate or histidine at pH 6.8–7.2 slow the loss to approximately 0.1 log₁₀ CFU/mL per day. Beyond 48 h, viable count loss increases rapidly. Final oral solutions are therefore not typically stored as multi-dose liquid forms unless an optimized stabilizing system is validated. Published data for this specific configuration is limited.