Antibiotic resistance (AR) represents a challenge for the treatment of infectious diseases. Traditionally, antibiotic resistance determinants have been retrieved from culturable bacteria which represent a minor fraction of the total microbial diversity found in natural environments such as soils. In this review, we summarize recent advances in the study of antibiotic resistance using two main culture-independent approaches: sequence-based metagenomics and functional metagenomics.

Antibiotic resistance has become a global issue. The most significant risk is the acquisition of these mechanisms by pathogenic bacteria, which can have a severe clinical impact and pose a public health risk. This problem assumes that bacterial fitness is a constant phenomenon and should be approached from an evolutionary perspective to develop the most appropriate and effective strategies to contain the emergence of strains with pathogenic potential. Resistance mechanisms can be understood as adaptive processes to stressful conditions. This review examines the relevance of homeostatic regulatory mechanisms in antimicrobial resistance mechanisms. We focus on the interactions in the cellular physiology of pathogenic bacteria, particularly Gram-negative bacteria, and specifically Klebsiella pneumoniae. From a clinical research perspective, understanding these interactions is crucial for comprehensively understanding the phenomenon of resistance and developing more effective drugs and treatments to limit or attenuate bacterial sepsis, since the most conserved adjuvant phenomena in bacterial physiology has turned out to be more optimized and, therefore, more susceptible to alterations due to pharmacological action.

Melanin is a pigmentated polymer with antioxidant capacity, and it also plays a role as a quality factor in those fungi that can synthetize it. However, this pigment is soluble only in alkaline solutions, limiting its application. This work used ultrasonication of fungal melanin, extracted from nonviable Cordyceps javanica CHE-CNRCB 307 conidia, and obtained hydrophilic melanin. Suspensions of freshly harvested conidia from Beauveria bassiana CHE-CNRCB 614 or Metarhizium acridum CHE-CNRCB 213 were prepared with a hydrophilic melanin solution at 0.1 mg/mL. These conidia formulations were more resistant to UV-B radiation, up to 34.9%, 26.2%, and 23.3% higher than those without hydrophilic melanin at doses of 10, 14, and 18 kJ/m², respectively, without affecting the virulence or thermotolerance parameters. Based on these results, hydrophilic melanin served as a compatible photoprotector in conidia suspensions from two widely entomopathogenic fungi used in biological control. In addition, the hydrophilic melanin source can be Cordyceps javanica conidia, which keeps melanin beyond the loss of viability, with high added value. Graphical abstract