Microbial Ecology in Health and Disease. 2009; 21: 178–182
ORIGINAL ARTICLE
Gut microbiota of crabs from the Vellar estuary, southeast coast
of India
GANAPATHY RAMESHKUMAR, SAMUTHIRAPANDIAN RAVICHANDRAN,
CHANDRASEKAR & THANGAPPAN AJITHKUMAR
Centre of Advanced Study in Marine Biology, Annamalai University, Parangipettai, India
Abstract
Animals in the aquatic environment carry a particular bacterial flora that is a reflection of their environment. In the present
study the level of total heterotrophic bacterial (THB) populations observed in the gut of crabs simply reflected the THB in
the water and sediment of the environments from which the crabs were collected. Crabs were collected from the different
areas, i.e. neritic, mangrove and oyster zones, of the Vellar estuarine environment of the southeast coast of India. THB
populations were high in the water and sediment in the mangrove and neritic zones. Among the three stations, less bacterial
populations were observed in the water, sediment and the gut of the crabs in the oyster zones due to the constant disturbance
of wave action from the estuarine mouth region. Bacteria belonging to the genera Pseudomonas and Vibrio were found to be
higher in all the three different stations. In the mangrove region, Bacillus was the dominant genus in sediment samples.
Key words: Gut microbiota, crabs, neritic zone, oyster zone, mangrove zone
Introduction
The gut serves as the natural habitat for innumerable
bacteria, some are beneficial to the host and others
are harmful. The primary function of the gut is to
take up water and nutrients. The specific role of the
resident colonic microbiota in digestion is to ferment
the substances provided in the diet (e.g. dietary fibre)
that cannot be digested by the host in the small intestine.
The role of microbiota as food, symbiont or competitor
in the nutrition of marine invertebrate detritivores is
not clearly understood. The gut microbiota of aquatic
invertebrates highlights the questions and processes
that merit acquisition of an understanding of the role
of gut microbes in the physiology of host invertebrates
and nutrient dynamics of aquatic systems. A number
of food products have been developed that can modify
the intestinal microbiota and possibly benefit health.
These contain probiotics, prebiotics and symbiotics
(a combination of probiotics and prebiotics). The role
of gut microbiota in the digestion of food of domestic herbivorores is well known. The gut microbiota
also plays an important role in the susceptibility of
marine deposit feeders. The biodiversity and in situ
abundance of the gut microbiota of abalone (Haliotis
discus hannai) as determined by culture-independent
techniques have been investigated (1,2). The gut
microbiota composition and the non-specific humoral
and cellular immune responses in rainbow trout
Oncorhynchus mykiss have been studied (3). The role
of gut microbiota and probiotic effects in irritable
bowel syndrome have also been studied (4).
The gut serves as the natural habitat for a
great number of bacteria – some beneficial to
the host, others harmful. The latter category includes
clostridia, sulphate reducers and proteolytic bacteria,
which are responsible for producing toxins that
can cause diarrhoea, mucosal invasion and carcinogenesis. Saccharolytic species, primarily bifid bacteria
and lactobacilli, are the main health-promoting
bacteria and are thought to be important barriers to
disease (5).
The amylase-producing ability of intestinal
bacteria in one mangrove crab and seven fish species
has been determined (6). Bacillus, Coryneforms,
Correspondence: Samuthirapandian Ravichandran, Centre of Advanced Study in Marine Biology, Annamalai University, Parangipettai 608502, India.
Tel: + 91 4144 243223, 243533. Fax: + 91 4144 243555. E-mail: [email protected]
(Received 16 June 2008; accepted 15 July 2009)
ISSN 0891-060X print/ISSN 1651-2235 online © 2009 Informa UK Ltd. (Informa Healthcare, Taylor & Francis AS)
DOI: 10.3109/08910600903194404
Gut microbiota of estuarine crabs
Enterobacteriaceae,
Flavobacterium,
Micrococcus,
Pseudomonas and Vibrio sp. were found in the intestinal tracts of Japanese flounder (7). It has also
been reported by several workers that fish of marine
origin harbor Vibrio sp. as the predominant intestinal
microbiota (8). However, such information on the gut
microbiota of estuarine crabs and their environment
has not been studied in detail, hence the present
study was carried out.
Material and methods
Samples were collected from the different areas,
namely neritic region, mangrove region and oyster
regions, of the Vellar estuary (Figure 1) during pre
and post monsoon seasons in 2006–2007 (Lat 11°
29´ N; 79°46´ E). The crab species were picked and
transferred to sterile polythene bags using sterile forceps. All the samples were transported immediately
to the laboratory and subjected to various analyses.
Water sample were collected from all the three
stations using sterile bottles (approximately 100 ml
from each site). Sediment samples were also collected from the same stations. Approximately 100 g
of the sediment were transferred aseptically to fresh
polythene bags using sterile spatula.
179
Enumeration of Total Heterotrophic Bacterial (THB)
population
The microbial loads were analysed by a serial dilution plating technique using Zobell Marine agar
medium (9). After homogenizing the collected water
sample, 1 ml of water sample was pipetted out using
a sterile pipette into a 99 ml blank and shaken well.
From this, 1 ml was pipetted out and added to the
9 ml blank, likewise the serial dilutions were used as
inocula. For a sediment sample, 1 g of sediment
from each sample was transferred aseptically to a
99 ml blank. The contents were homogenized for
10 min.
From this, 1 ml was transferred aseptically to a
9 ml blank and mixed thoroughly. Similarly serial
dilutions were made and used as inocula. For gut
analysis, the digestive system was dissected out aseptically using sterile scissors and forceps. Guts were
homogenized in a tissue homogenizer and transferred
to a 99 ml blank, from the suspensions, 1 ml of sample
was pipetted out and added to a 9 ml blank; likewise
serial dilutions were made. For all the samples triplicate plates were inoculated at a temperature of 28 1°C
for 2–7 days and the colonies were counted. The
microbial load was counted and expressed as the
number of colony forming units (CFU). The bacteria
were identified to generic level (10,11).
Results
THB load in water samples
The THB count in water from three different estuarine environments is shown in Figure 2. Among the
three zones, a high bacterial load in water was
recorded in the mangrove region (8.70 105) followed by the neritic zone (5.20 103) and the oyster
zone (1.30 107). The oyster zone, which had much
9.00
THB Value (CFU)
8.00
Neritic Zone
Mangrove Zone
Oyster Zone
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
1
Different Zones
Figure 1. Location of the sampling sites along the Vellar estuary.
Figure 2. THB load in water sample.
180
G. Rameshkumar et al.
wave influence, showed much lower counts of THB
than the mangrove region.
Neritic Zone
Mangrove Zone
Oyster Zone
8.00
The THB count in sediment collected from three
different environments is shown in Figure 3. For the
sediment, a high THB load was noted in the mangrove
region (2.02104) and a lower bacterial count
(1.50 104) in the neritic zone region.
THB value (CFU)
7.00
THB load in sediment samples
6.00
5.00
4.00
3.00
2.00
1.00
0.00
THB load in the gut of Scylla serrata
The THB counts in the gut of S. serrata collected
from three different stations are shown in Figure 4.
S. serrata collected from the three different regions
showed a high THB load (7.80 105) in the gut of
crabs collected from the neritic zone.
Comparison of different crabs for gut microbiota
Comparison of the THB count in the gut region of five
economically important crabs is shown in Figure 5.
The THB load varied in the guts of crab species investigated. The highest THB load was noted in the
gut of mud crabs Scylla serrata (7.80 105) and S.
tranquebarica (5.40 105) followed by swimming
crab Charybdis helleri (4.20 105), mosac crab Portunus pelagicus (3.70 105) and three eye spot crab P.
sanguinolentus (2.30 105). The count in other crabs
was shown to be much lower than in the mud crab
species. The lowest count was noted in the gut of
P. sanguinolentus (2.30 105). The bacterial genera
isolated from the water, crabs and sediment samples were Bacillus, Micrococcus, Coryneforms, Vibrio,
Pseudomonas, Aeromonas, Achrobacter, Flavobacterium
and Enterobacterium. The bacterial flora collected
from the gut region of crabs is a reflection of their
environment.
1
Different Zones
Figure 4. THB load in the gut of Scylla serrata.
Discussion
The phylogenetic history of decapods plays an important role in the gross structure of the foregut. It is clear
that bacterial species in the gut can influence the
health and robustness of the host. Extreme examples
of the influence of the gut flora include the negative
effects of pathogenic organisms and, in contrast, the
total reliance that ruminants have on their gut flora
for the assimilation of organic carbon from the environment (12). More subtle interactions have usually
been much less characterized and understood but
are, nevertheless, important and have been shown to
impact on the health and growth of the host (13).
The influence of the gut flora on the host is clearly
of great interest in aquaculture, particularly where
poor productivity and/or stock losses are widespread
(1). Within marine and other aquatic marine animals,
the colonization of the digestive system by microorganisms is influenced by a number of both host-related
and non-host-related factors (14).
8
Neritic Zone
Mangrove Zone
Oyster Zone
2.00
1.50
1.00
0.50
0.00
Scylla tranquebarica
7
Portunus pelagicus
THB Value (CFU)
THB Value (CFU)
2.50
Scylla serrata
6
Portunus sanguinolentus
5
Charybdis helleri
4
3
2
1
1
Different Zones
Figure 3. THB load in sediment sample.
0
1
Crabs species
Figure 5. Comparison of gut microbiota of different crabs.
Gut microbiota of estuarine crabs
Within a natural environment, conditions may
lead to the development of stable populations of gut
flora, which may represent the normal flora of the host
animal (15). The degradation of litter to particulate
organic matter and to fine detritus is a complex process
of ecological energetic. The microorganisms convert
substances such as cellulose and lignin present in the
mangrove leaves into digestible matter, which is utilized by the animal communities (16,17). In general,
the initial decomposition is carried out by microorganisms like bacteria, followed by higher organisms
such as crabs, and the energy is transferred to higher
trophic levels.
Animals in the aquatic environment carry a
particular bacterial flora, which is a reflection of their
environment. The levels of THB observed in the
guts of crabs simply reflected the THB in the water
and sediment of the environments from which the
crabs were collected. But the sediment and water of
the marine environment carry lower populations of
bacteria than the alimentary tracts of animals (18).
The digestive tracts of most of the marine animals
contain a commensal microbiota (19). The role of
the gut microbiota in the digestion of food in herbivorous is well known. The gut microbiota plays a
vital role in the digestive process, growth and disease
susceptibility of marine deposit feeders (20).
Bacteria living in the gut region have the ability
to digest the carbohydrates (21). Further, the midgut
of crabs supports good growth of proteolytic bacteria
(22). Moreover, in comparison with the diverse bacterial population that has been suggested to occur in
healthy individuals (under normal conditions) (23),
the digestive system of unhealthy or diseased prawns
may be found to support a gut flora where only one
or two bacterial species predominate (24). In the
present study, a higher level of bacterial load was
observed in the gut of S. serrata and a lower level in
C. helleri and P. pelagicus. Thus the bacterial load varied
with feeding habits. S. serrata has a high level of gut
bacterial flora due to the carnivorous feeding habitat
of the mollusc Buillia vittata.
Generally, the tropical mangrove ecosystem is
endowed with a high bacterial load due to the continuous shedding of foliage into the water and subsequent
decomposition (25). THB populations were high in the
water and sediment in the mangrove region and neritic
zone. Among the three stations studied, smaller bacterial populations were observed in the water, sediment
and the gut of the crabs in the oyster region due to the
constant disturbance of wave action from the estuarine
mouth region. Bacteria belonging to the genera
Pseudomonas and Vibrio were found to at higher levels
in all the three different stations. In the mangrove
region, Bacillus was the dominant genus in sediment
samples. In conclusion, crabs in the various estuarine
181
environments carry a particular bacterial flora, which is
a reflection of their environment.
Acknowledgements
The authors are grateful to the Director of CAS in
Marine Biology and authorities of Annamalai University, and to the Centre for Marine Living Resources and
Ecology (CMLRE), Government of India, Cochin, for
the financial support, and staff members of ENVIS
centre for their keen interest and encouragement.
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