## 3 CA Production in _S. clavuligerus_ Submerged Cultivation
Other factors apart from culture media could affect the CA accumulation in submerged cultures of _S. clavuligerus_. The CA molecule shows high susceptibility to temperature and ionic strength in aqueous solutions, compromising the CA titers during the cultivation [67; 68]. The pH conditions seem to affect CA yield, possibly more linked to molecule degradation than biosynthesis inhibition [69]. The highest concentration of CA has been reported at neutral or slightly acidic pH conditions (6.8), regardless of the substrates used in the cultivations [55]. A recent report showed that STR cultures of _S. clavuligerus_ subjected to acid stresses (pH reduction from 6.8 to 6.3 at a rate of 0.1 pH every 6 h) may lead to higher global yields (Y\({}_{\text{CA/X}}=0.851\) g\({}_{\text{CA}}.\)g\({}_{\text{X}}^{-1}\)) [70]. Low temperatures favor themolecule stability; in this regard, higher titers have been reported during operating fermentations at 20 \({}^{\circ}\)C but the growth rate at such temperature is lower than that observed at 28 \({}^{\circ}\)C [70; 71]. In addition, it has been found that both bacterial morphology in dispersed secondary mycelium and a high density spore suspension inoculum (with concentration \(\geq\)5\(\%\)) significantly improve CA biosynthesis [72; 73; 74]. Regarding the operational variables associated to fermentative processes in _Streptomyces_ species, the nutritional effects have been more studied than the effects of hydrodynamic conditions and reactor geometry [75]. The conventional bioprocesses with _Streptomyces_ strains are generally carried out in STRs, as this traditional geometry has been proven to be reliable as it assures good mixing, mass, and heat transfer rates [76]. Novel impeller geometries for STR have been studied as feasible alternatives for improving the oxygen dissolution and nutrient dispersion during the fermentative process [77]. Few studies involving the hydrodynamic patterns of reactors and its impact on growth, morphology, and mass transfer in _Streptomyces_ cultures have been performed. Such studies are required for a more precise description of antibiotics biosynthesis in bioreactors [75]. The reactor geometry, impeller configuration, and velocity of mixing may impact the oxygen availability on the system; it has been demonstrated that higher mixing velocities and turbulence gives better volumetric oxygen mass transfer coefficients (\(k_{La}\)), promoting higher specific growth rates [76; 78]. The reactor design must be focused on balancing mass and heat transfer with shear rates, as the latter can cause hydrodynamic stress on cells, affecting their growth and productivity. In this direction, the novel single-use technologies can offer some advantages, in addition to the better sterility conditions attained in this kind of reactors [75]. In the operation of bioreactors, the agitation rate is related not only with the dissolved oxygen and mass transfer conditions, but also with the shear conditions. The shear forces may affect the broth rheology and transport of nutrients as well as the growth rate and morphology expression on _Streptomyces_ species [79; 80]. In the case of _S. clavuligerus_, agitation and aeration are also considered as factors potentially affecting the CA production. An increase of 50% in oxygen transfer in shake flask cultures increased the CA production 2-fold [81]. Regarding the bioreactor operation, [PERSON] et al. [82] found that intense agitation rates, i.e., 800-1000 rpm, favored CA accumulation in STRs. The \(k_{La}\) observed in airlift reactors, operated in the range from 3 to 5 vvm, are rather comparable to those arising in STRs operating in the range of 600 to 1000 rpm; both conditions promote CA accumulation. An analysis of the _S. clavuligerus_ response to shear stress conditions showed that high shear forces (up to 7.5 Pa.s) increase the growth rate, mycelial thinning, and CA production, in comparison with cultivations at low shear forces (<1 Pa.s) [79].