Chapter 4 :Bacterial Nutrition

Driven by a mysteriousmechanism, the bacteriumgrows and divides in approximatelytwenty minutes, givingrise to twodaughterbacteriathatinheritfrom the mothercell the samepotential for activity, the samecomplex structures, and the sameimperativeneed for division. Thus, to thoroughlystudy and understand the phenomena of bacterial nutrition, itis essential to comprehend the elemental and energeticrequirementsnecessary for bacterialgrowth, as well as the physicochemicalfactorsthat influence it.

1. Energy and ElementalRequirements

Bacteriamultiplyusingnutrientspresent in culture media. They all share a number of commonneeds: water, an energy source, acarbon source, anitrogen source, and mineralelements. Whilemanycangrowunderthese conditions, others are completely incapable because an essential metaboliteismissing and must beadded to the medium to ensuredevelopment—thisiscalled a growth factor.

1.1. Energy Source

Based on theirenergy source, bacteria can be divided in to two major classes :

A. Phototrophs or PhotosyntheticBacteria:Thesederivetheirenergyfrom light radiation. Amongphotosyntheticbacteriaare :

Photo-lithotrophicBacteria:Thesebacteriadegradeinorganicsubstrates, such as sulfur (e.g.,sulfurbacteria) (electron source).

Photo-organotrophicBacteria:Thesedegradeorganic compounds likefattyacids, lipids, proteins, etc. (electron source).

B. Chemotrophs or ChemosyntheticBacteria:These use energyfrom the oxidation of chemicalcompound, whetherorganic or inorganic. Amongchemotrophicbacteria, thereare :

ChemolithotrophicBacteria:Thesedegradeinorganicsubstrates.

Chemo-organotrophicBacteria:Thesedegradeorganiccompounds, with the electron source beinginorganic or organic, respectively.

1.2. ElementalRequirements

1.2.1. Carbon Source

Carbonis a highlyabundantelement in bacterialcells and must besupplied in large quantities. It isnoweasy to identify the substances that serve as carbon sources for bacteria. Theserequirementsgenerally lead to two major categories of microorganisms:

Those capable of growing in an inorganic medium containing CO₂ as the sole carbon source (autotrophicbacteria).

Othersthatrequireorganic compounds to grow and reproduce, commonlyreferred to as heterotrophicbacteria.

1.2.2. Nitrogen Source

To synthesizetheirproteins, whichaccount for about 10% of their dry weight, bacteriaalwaysneednitrogenous substances. Few bacteriacanfixnitrogen in itsmolecularform, such as Rhizobium (which live in symbiosiswithlegumes), Azotobacter, and someClostridiumspecies. Others, likeNitrosomonas, use ammonium salts. Nitrogencanalsobesourcedfromorganic compounds, such as amino groups, whichcanbeincorporated via transamination.

1.2.3. Sulfur and Phosphorus

Among the mineralelements in bacteria, sulfur and phosphorusholdaprominent place. Sulfurispresent in certain aminoacids and thus in proteins as thiol groups (-SH). It isprimarilyincorporated in reducedform. Phosphorusis part of nucleicacids, many coenzymes, and ATP. It isincorporatedinto the cell as inorganic phosphate and serves as an energy hub at the cellular level, enabling the recovery, accumulation, and distribution of energywithin the cell.

1.2.4. OtherMineralElements

A large number of chemicalelementsplay a role in the physicochemical balance of the cell, such as sodium, potassium, magnesium, and chlorine. Others are integral parts of enzymes or coenzymes, such as iron in cytochromes and magnesium in chlorophyll. Additionalelementslike calcium, cobalt, copper, manganese, molybdenum, and vanadium act as cofactors or enzymaticactivators. These are generallycalledtrace elementsbecausethey are indispensable in minute quantities.

2. SpecificRequirements—GrowthFactors

Growthfactors are organic substances essential for growththat the bacteriumcannotsynthesize. The concept of growthfactorsiscloselyrelated to that of essential metabolites, which are organiccompoundderivedfrombiosyntheticpathways and strictlynecessary for bacterialdevelopment. Based on thesefactors, microorganisms are dividedintotwo major categories:

PrototrophicMicroorganisms:These do not requiregrowthfactors; the usualelementsmentionedearliersuffice.

AuxotrophicBacteria:Theserequiregrowthfactors.

2.1. Classification and Properties

Growthfactorsencompassthreecategories of substances :aminoacids, purine and pyrimidine bases, and vitamins.

2.2. Mode of Action

Aminoacids are growthfactorsbecausethey are essential for proteinsynthesis. Purine and pyrimidine bases are part of nucleicacids and are necessary for their formation. Vitamins are either coenzymes or coenzyme precursors, and their absence deprivesbacteria of the correspondingfunctions.

2.3. Syntrophy

The growth factor requirements of amicrobialspeciescansometimesbe met by the presence of anotherspeciesthatsynthesizes the factor. This metabolic interaction phenomenoniscalledsyntrophy. It isillustrated by the presence of satellite colonies, whichoftendeveloponsolid media near a colonyproducingvitamins.

3. Physical Factors

The nutrients or energy sources required for bacterial nutrition must beprovidedunderspecificenvironmental conditions. Severalphysicalfactors influence nutrition, eitherinhibiting or promotingit.

3.1. Temperature

Temperatureprofoundly affects microbial multiplication and metabolism. Based on optimal growthtemperatures, microorganisms are generallydividedintothreecategories:

Mesophiles:Prefermoderatetemperaturesbetween 20 and 40°C.

Psychrophiles : Optimal growthtemperatureisaround 0°C.

Thermophiles :Prefertemperaturesbetween 45 and 65°C.

This classification is not strict, and overlapsmayexist. For example, somemesophilescanalsobe thermophiles. Othertermsincludepsychrotrophs (grow at refrigerationtemperatures but prefer 10–30°C) and thermotrophs (growvisibly at 50°C but more clearly at moderatetemperatureslike 30°C). However, mostmicroorganisms are mesophiles.

3.2. PH

Unlikemolds and yeasts, whichpreferacidic pH (3–6) for growth, mostbacteriamultiply in neutral pH (7–7.5). Some, likeE. coli, tolerateabroader range (4.4–8). Bacteria are generallydividedinto:

Acidophiles :Preferslightlyacidic pH (e.g.,Thiobacillusthiooxidans at pH 2, Lactobacillus at pH 6).

Alkaliphiles:Tolerate or thrive in high pH (e.g.,Vibrio at pH 9).

Culture media are usuallysuitable for bacterialgrowth, but acidification or alkalizationfrombacterialmetabolitescanhindergrowth. Buffers like K₂HPO₄ and KH₂PO₄ are commonlyused to stabilize pH.

3.3. GasRequirements

Bacterialgasrequirements are mostpreciseregardingoxygen. Someare :

Strict Aerobes:Require free oxygen for growth.

Strict Anaerobes:Growonly in the absence of free oxygen.

Facultative Anaerobes: Can growwith or without free oxygen.

Microaerophiles:Growonly in lowoxygen concentrations.

Aerophiles:Grow at the surface of liquid media, forming a film.

3.4. Osmotic Pressure

Osmotic pressure isdirectlyproportional to the concentration of ions and molecules in solution. When the concentration isequalinside and outside the bacterialcell, the medium isisotonic. If higher or lower, itishypertonic or hypotonic, respectively. Most bacteria are insensitive to osmotic pressure variations due to theirrigidcellwalls. However, marine bacteria (halophiles) thrive in high salt concentrations (e.g., 35g NaCl per liter) and require media with at least 10‰ salt.