This document discusses various types of xenobiotics (foreign chemicals) including pesticides, hydrocarbons, plastics, and other industrial chemicals. It describes their sources and outlines several mechanisms by which microorganisms can biodegrade these compounds, including hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Specific pathways and microbes involved in degrading compounds like polychlorinated biphenyls, polycyclic aromatic hydrocarbons, and various plastics and pesticides are also summarized.

2. XENOBIOTICS
 It is derived from a greek word “XENOS” meaning ‘foreign
or strange’.
 Xenobiotics are usually those chemicals which are man-
made.
 They are usually synthesized for industrial or agricultural
purposes e.g. aromatics, pesticides, hydrocarbons,
plastics , lignin etc.
 They are also called RECALCITRANTS as they can resist
degradation to maximum level.

3. biodegradation
 According to the definition by the International Union of
Pure and Applied Chemistry, the term biodegradation is
“Breakdown of a substance catalyzed by enzymes in vitro
or in vivo.
 In other words, defined as the ability of microorganisms to
convert toxic chemicals (xenobiotics) to simpler non-toxic
compounds by synthesis of certain enzymes
 Biodegradation of xenobiotics can be affected by
substrate specificity, nutrition source, temperature, pH
etc.

4. Sources of xenobiotics
1. Petrochemical industry :
- oil/gas industry, refineries produces basic chemicals
e.g. vinyl chloride and benzene
2. Plastic industry :
- closely related to the petrochemical industry
- uses a number of complex organic compounds
- such as anti-oxidants, plasticizers, cross-linking agents

5. BIODEGRADATION OF PESTICIDES
 Pesticides are substances meant for destroying or
mitigating any pest.
 They are a class of biocide.
 The most common use of pesticides is as plant protection
products (also known as crop protection products).
 It includes: herbicide, insecticide, nematicide, termiticide,
molluscicide, piscicide, avicide, rodenticide, insect
repellent, animal
repellent, antimicrobial, fungicide, disinfectant,
and sanitizer.

6. 3. Pesticide industry :
- most commonly found structures are benzene and
benzene derivatives.
4. Paint industry :
- major ingredient are solvents,
xylene, toluene, methyl ethyl ketone.
5. Others :
- Electronic industry, Textile industry, Pulp and Paper
industry, Cosmetics and Pharmaceutical industry, Wood
preservation

7. DIFFERENT METHODS for degradation
a) Detoxification:
 Conversion of the pesticide molecule to a non-toxic compound.
 A single moiety in the side chain of a complex molecule is
disturbed(removed), rendering the chemical non-toxic.
b) Degradation:
 Breakdown or transformation of a complex substrate into
simpler products leading to mineralization.
 E.g. Thirum (fungicide) is degraded by a strain of
Pseudomonas and the degradation products are dimethylamine,
proteins, sulpholipids, etc (Raghu et al., 1975).

8. c) Conjugation (complex formation or addition reaction):
 An organism makes the substrate more complex or
combines the pesticide with cell metabolites.
 Conjugation or the formation of addition product is
accomplished by those organisms catalyzing the reaction
of addition of an amino acid, organic acid or methyl crown
to the substrate thereby inactivating the pestcides
d) Changing the spectrum of toxicity:
 Some pesticides are designed to control one particular
group of pests, but are metabolized to yield products
inhibitory to entirely dissimilar groups of organisms, for
e.g. the fungicide PCNB is converted in soil to chlorinated
benzoic acids that kill plants.

9. There are many mechanisms involved on the biodegradation of pesticides
and other contaminants. These may be summarised as follows:
Dehalogenation- nitrofen, DDT, cyanazine, propachlor.
Deamination- fluchloralin
Decarboxylation- DDTc, biofenox, dichlorop-methyl
Methyl oxidation- bromacil
Hydroxylation- benthiocarb, bux insecticide

10. BIODEGRADTION OF PLASTICS
 Plastic is a broad name given to different polymers with high
molecular weight, which can be degraded by various processes.
 The biodegradation of plastics by microorganisms and enzymes
seems to be the most effective process.
 It consist of two steps- fragmentation and mineralization. But at
the core, reaction occurring at molecular level are oxidation and
hydrolysis.
 The decomposition of major condensation polymers (e.g.
polyesters and polyamides) takes place through hydrolysis,
while decomposition of polymers in which the main chain
contains only carbon atoms (e.g. polyvinyl alcohol, lignin)
includes oxidation which can be followed by hydrolysis of the
products of oxidation.

11. The four stages of anaerobic digestion:
•Hydrolysis: A chemical reaction where large polymers converted into
simpler monomers
•Acidogenesis: A biological reaction where simple monomers are
converted into volatile fatty acids
•Acetogenesis: A biological reaction where volatile fatty acids are
converted into acetic acid, carbon dioxide, and hydrogen
•Methanogenesis: A biological reaction where acetates are converted
into methane and carbon dioxide, while hydrogen is
consumed.

12. METHOD
HYDROLYSIS-
 The process of breaking these chains and dissolving the polymers into
smaller fragments is called hydrolysis. E.g. Pseudomonas spp.
 Polymeric Chains is broken down into constituent parts for the energy
potential by microorganisms. Monomers are readily available to
other bacteria and is used.
 Acetate and hydrogen produced is used directly by methanogens.
Other molecules, such as volatile fatty acids (VFAs) with a chain
length greater than that of acetate is first catabolized into compounds
that can be directly used by methanogens.
ACIDOGENESIS-
 This results in further breakdown of the remaining components by
acidogenic (fermentative) bacteria into ammonia, ethanol, carbon
dioxide, and hydrogen sulfide. E.g Streptococcus acidophilus.

13. ACETOGENESIS-
 Simple molecules created through the acidogenesis phase
are further digested by Acetogens to produce largely
acetic acid, as well as carbon dioxide and hydrogen.
METHANOGENESIS-
 Here, methanogens use the intermediate products of the
preceding stages and convert them into methane, carbon
dioxide, and water.
 These components make up the majority of the biogas
emitted.
 Methanogenesis is sensitive to both high and low pHs and
occurs between pH 6.5 and pH 8. The remaining,
indigestible material the microbes cannot use and any
dead bacterial remains constitute the digestate.

14. Some of the microorganism that can degrade plastics are:-
Aliphatic Polyesters
PolyEthylene Adipate (PEA)- lipases from R. arrizus, R. delemar,
Achromobacter sp. and Candida cylindracea
Poly (β-Propiolactone) PPL - estereases
from Acidovorax sp., Variovorax paradoxus, Sphingomonas
paucimobilis.
Aromatic Polyesters
Poly-3-Hydroxybutyrate (PHB) – estereases from Pseudomonas
lemoigne, Comamonas sp. Acidovorax faecalis, Aspergillus
fumigatus
Poly Lactic Acid (PLA) - proteinase K from Tritirachium album,
Amycolatopsis sp
Strains of Actinimycetes has been reported to degrade polyamide
(nylon), polystyrene, polyethylene.

15. BIODEGRADATION OF
HYDROCARBONS
 A hydrocarbon is an organic compound consisting
entirely of hydrogen and carbon.
 The majority of hydrocarbons found on earth naturally
occur in crude oil.
 Aromatic hydrocarbons (arenes), alkanes,
alkenes, cycloalkanes and alkyne-based compounds
are different types of hydrocarbons.

16. BIODEGRADATION OF
PETROLEUM
 Petroleum compounds are categorized into 2 groups
1. Aliphatic hydrocarbon e.g. alkane, alcohol, aldehyde
2. Aromatic hydrocarbon e.g. benzene, phenol, toluene,
catechol
 Aromatic hydrocarbons are degraded aerobically and
anaerobically.

17. AEROBIC DEGRADATION
 Are metabolized by a variety of bacteria, with ring
fission.
 Accomplished by mono- and dioxygenases.
 Catechol and protocatechuate are the intermediates.
 Mostly found in aromatic compound degradation
pathway.

18. 18
OTHER MECHANISMS
1) Photometabolism : in bacteria, this light-induced
“bound oxygen” (OH•
) is used to oxidize substrates.
2) under nitrate-reducing condition : Nitrate-reducing bacteria
couple the oxidation of organic compound with water to the
exergonic reduction of nitrate via nitrite to N2..
3) dissimilation through sulfate respiration: Sulfate-
reducing bacteria couple the oxidation of organic compound
with water to the exergonic reduction of sulfate via sulfite to
sulfide.

19. 19
Genetic Regulation of Xenobiotic Degradation
plasmid-borne
mostly in the genus Pseudomonas
PLASMID SUBSTRATE
TOL Toluene, m-xylene, p-xylene
CAM Camphor
OCT Octane, hexane, decane
NAH Napthalene
pJP1 2,4-Dichlorophenoxy acetic acid
pAC25 3-Chlorobenzoate
SAL Salicylate

20. POLYCYCLIC AROMATIC
HYDROCARBONS (PAH)
 Bacteria, fungi, yeasts, and algae have the ability to
metabolize both lower and higher molecular weight PAHs
found in the natural environment.
 Most bacteria have been found to oxygenate the PAH
initially to form dihydrodiol with a cis-configuration, which
can be further oxidized to catechols.
 Most fungi oxidize PAHs via a cytochrome P450 catalyzed
mono-oxygenase reaction to form reactive arene oxides
that can isomerize to phenols.
 White-rot fungi oxidize PAHs via ligninases (lignin
peroxidases ) to form highly reactive quinones.

21. POLYCHLORINATED
BIPHENYLS (PCBs)
 Synthesized chemicals from petro-chemical industry
used as lubricants and insulators in heavy industry.
 First manufactured in 1929 by Monsanto.
 Manufacture and unauthorized use banned in 1978 by
USEPA
 Used because-
 Low reactivity
 Non-flammable
 High electrical resistance
 Stable when exposed to heat and pressure
 Used as Hydraulic fluid, Casting wax, Carbonless carbon
paper, Compressors, Heat transfer systems, Plasticizers,
Pigments, Adhesives, Liquid cooled electric motors,

22. RISKS-
 Causes reproductive disabilities in animals, human,
birds.
 Carcinogenic
 Bioaccumulation
 Soluble in almost all the solvents, fats, oils
 Nervous system damage
 Endocrine gland malfunction

23. Methods for PCB removal
 Natural Attenuation: Microbes already in the soil are
allowed to degrade as they can naturally and the
site is closely monitored.
 Biostimulation: Microbes present in the soil are
stimulated with nutrients such as oxygen, carbon
sources like fertilizer to increase degradation.
 Bioaugmentation: Microbes that can naturally
degrade PCB’s are transplanted to the site and fed
nutrients if necessary.

24. Pathways for pcb removal
Fungal DEGRADATION –
 Aspergillus niger: fillamentous with cytochrome p450
that attacks lower chlorinated PCB’s
 Phanerochaete chrysosporium: White rot fungi can
attack lignin (PCB) at low concentration with the help
of ligninases.
BACTERIAL degradation-
 Soil bacteria breaks down PCBs via dioxygenase
pathways.
 Most identified seem to be Pseudomonas species,
Achromobacter, Acinetobacter, Alcaligenes,

25. reference
BOOKS-
 Sullia S.B and Shantharam S.; General microbiology, Second
editionPage No. 348-350
 Dubey R. C and Maheshwari D.K; A textbook of microbiology,
second revised edition 2009; Page No. 832-836
ARTICLES
 Biodegradation of polymers- Dr Rolf Joachim Miller
 Biodegradation of pesticides- Andre Luiz Porto, Marcia Nitcshke
and Gleiseda Melgar
 Microbial Degradation of Petroleum Hydrocarbon -Nilanjana Das
and Preethy Chandran

26. .