Microbes break down lipids and proteins to fuel their growth and survival. This process involves enzymes that chop up complex molecules into simpler ones, which can then be used for energy or to build new cell parts.

Different microbes have unique ways of breaking down these nutrients. This helps scientists identify them. For example, some bacteria can digest a wide range of lipids, while others need special enzymes to break down their own cell walls.

Lipid Catabolism in Microorganisms

Lipid catabolism in microorganisms

• Lipid catabolism breaks down lipids to generate energy for microorganisms
  • Lipases hydrolyze lipids into their constituent parts glycerol and fatty acids
  • Glycerol is transformed into dihydroxyacetone phosphate (DHAP) which then participates in either glycolysis or gluconeogenesis pathways
  • Fatty acids are degraded through β-oxidation, a repetitive process that cleaves two-carbon units (acetyl-CoA) from the fatty acid chain
    • Acetyl-CoA molecules feed into the citric acid cycle where they are further oxidized to produce energy
• Fatty acids must first be activated by the enzyme acyl-CoA synthetase to form fatty acyl-CoA before lipid catabolism can proceed
• In most bacteria, β-oxidation takes place in the cytoplasm, contrasting with eukaryotic cells where it occurs within mitochondria
• Lipolysis, the breakdown of triglycerides into free fatty acids and glycerol, is the initial step in lipid catabolism

Microbial identification through lipid breakdown

• Microorganisms differ in their ability to break down various types of lipids
  • Certain bacterial genera like Pseudomonas and Acinetobacter can catabolize a broad spectrum of lipids, even complex hydrocarbons
  • In contrast, bacteria such as Mycobacterium tuberculosis have lipid-rich cell walls that require specialized lipases for degradation
• Lipid degradation patterns serve as useful diagnostic markers for identifying specific microbes
  • As an example, detecting lipase activity and the capacity to break down particular lipids helps differentiate between various Pseudomonas species (P. aeruginosa, P. fluorescens)

Protein Catabolism in Bacteria

Protein catabolism in bacteria

• Bacterial protein catabolism involves breaking down proteins into their constituent amino acids, which are then further catabolized for energy or used in biosynthetic pathways
• The catabolism process is initiated by extracellular proteases that hydrolyze peptide bonds, yielding smaller peptides and free amino acids
• Peptides are then transported into the bacterial cell via specific peptide transport systems (Opp, Dpp)
• Once inside the cell, peptidases continue to degrade the peptides into individual amino acids
• Amino acids can either be directly incorporated into bacterial proteins or undergo deamination, which removes the amino group as ammonia and generates the corresponding α-keto acid
  • These α-keto acids can then enter the citric acid cycle to produce energy or be used to synthesize other cellular compounds (fatty acids, sugars)
• Transamination reactions play a crucial role in amino acid metabolism, transferring amino groups between amino acids and α-keto acids

Bacterial differentiation via protein degradation

• Different bacterial species possess varying capacities for protein degradation, which can be exploited to differentiate between them
  • Some bacteria, like Bacillus subtilis, secrete a wide array of proteases enabling them to efficiently break down many different proteins
  • Others, such as Streptococcus pyogenes, have a more limited set of proteases and may depend on specific peptide transport systems for acquiring nutrients
• The presence or absence of particular proteases and peptidases serves as a diagnostic tool for identifying bacterial species
  • For example, the ability to produce specific extracellular proteases like collagenase distinguishes Clostridium histolyticum from other Clostridium species (C. difficile, C. botulinum)

Alternative Metabolic Pathways

Ketogenesis in microorganisms

• Some microorganisms can utilize ketogenesis, a metabolic pathway that produces ketone bodies from fatty acids when glucose is limited
• Ketogenesis occurs in the mitochondria and involves the breakdown of acetyl-CoA derived from fatty acid oxidation
• The main ketone bodies produced are acetoacetate, β-hydroxybutyrate, and acetone
• Ketogenesis is particularly important in certain anaerobic bacteria and archaea, allowing them to thrive in environments where carbohydrates are scarce