228 Modern Food Microbiology
contaminates a surface needs to contain at least 10 cells per milliliter before results can be achieved
either by contact or by swabs.
87 The latter investigators found that the contact plate removed only
about 0.1% of surface biota. This suggests that 10 cfu/cm
2 detected by this method are referable to
a surface that actually contains about 10
4 cfu/cm
2 . When stainless-steel surfaces were contaminated
by B. subtilis endospores, 41% were recovered by the RODAC plate compared to 47% by the swab
method.
7 In another study, swabs were better than contact plates when the contamination level was
100 or more organisms per 21–25 cm
2 .
112 On the other hand, contact plates give better results where
low numbers exist. In terms of ranking of surface contamination, the two methods correlated well.
Agar Syringe/“Agar Sausage” Methods
The agar syringe method was proposed by W. Litsky in 1955 and subsequently modified.
6 By this
method, a 100-ml syringe is modified by removing the needle end to create a hollow cylinder that is
filled with agar. A layer of agar is pushed beyond the end of the barrel by means of the plunger and
pressed against the surface to be examined. The exposed layer is cut off and placed in a Petri dish,
followed by incubation and colony enumeration. The “agar sausage” method proposed by ten Cate
125
is similar but employs plastic tubing rather than a modified syringe. The latter method has been used
largely by European workers for assessing the surfaces of meat carcasses, as well as for food plant
surfaces. Both methods can be viewed as variations of the RODAC plate, and both have the same
disadvantages: spreading colonies and applicability limited to low levels of surface contaminants.
Because clumps or chains of organisms on surfaces may yield single colonies, the counts obtained by
these methods are lower than those obtained by methods that allow for the breaking up of chains or
clumps.
For the examination of meat carcasses, Nortje et al.
88 compared three methods: a double swab,
excision, and agar sausage. Although the excision method was found to be the most reliable of the
three, the modified agar sausage method correlated more closely with it than the double swab, and the
investigators recommended the agar sausage method because of its simplicity, speed, and accuracy.
Other Surface Methods
Direct Surface
A number of workers have employed direct surface agar plating methods, in which melted agar is
poured onto the surface or utensil to be assessed. Upon hardening, the agar mold is placed in a
Petri dish and incubated. Angelotti and Foter
6 proposed this as a reference for assessing surface
contamination, and it is excellent for enumerating particulates containing viable microorganisms.
35 It
was used successfully to determine the survival of Clostridium sporogenes endospores on stainlesssteel surfaces.
83 Although effective as a research tool, the method does not lend itself to routine use
for food plant surfaces.
Sticky Film
The sticky film method of Thomas has been used with some success by Mossel et al.
82 The method
consists of pressing sticky film or tape against the surface to be examined and pressing the exposed
side on an agar plate. It was shown to be less effective than swabs in recovering bacteria from wooden
surfaces.
82 An adhesive tape method has been employed successfully to assess microorganisms on meat
contaminates a surface needs to contain at least 10 cells per milliliter before results can be achieved
either by contact or by swabs.
87 The latter investigators found that the contact plate removed only
about 0.1% of surface biota. This suggests that 10 cfu/cm
2 detected by this method are referable to
a surface that actually contains about 10
4 cfu/cm
2 . When stainless-steel surfaces were contaminated
by B. subtilis endospores, 41% were recovered by the RODAC plate compared to 47% by the swab
method.
7 In another study, swabs were better than contact plates when the contamination level was
100 or more organisms per 21–25 cm
2 .
112 On the other hand, contact plates give better results where
low numbers exist. In terms of ranking of surface contamination, the two methods correlated well.
Agar Syringe/“Agar Sausage” Methods
The agar syringe method was proposed by W. Litsky in 1955 and subsequently modified.
6 By this
method, a 100-ml syringe is modified by removing the needle end to create a hollow cylinder that is
filled with agar. A layer of agar is pushed beyond the end of the barrel by means of the plunger and
pressed against the surface to be examined. The exposed layer is cut off and placed in a Petri dish,
followed by incubation and colony enumeration. The “agar sausage” method proposed by ten Cate
125
is similar but employs plastic tubing rather than a modified syringe. The latter method has been used
largely by European workers for assessing the surfaces of meat carcasses, as well as for food plant
surfaces. Both methods can be viewed as variations of the RODAC plate, and both have the same
disadvantages: spreading colonies and applicability limited to low levels of surface contaminants.
Because clumps or chains of organisms on surfaces may yield single colonies, the counts obtained by
these methods are lower than those obtained by methods that allow for the breaking up of chains or
clumps.
For the examination of meat carcasses, Nortje et al.
88 compared three methods: a double swab,
excision, and agar sausage. Although the excision method was found to be the most reliable of the
three, the modified agar sausage method correlated more closely with it than the double swab, and the
investigators recommended the agar sausage method because of its simplicity, speed, and accuracy.
Other Surface Methods
Direct Surface
A number of workers have employed direct surface agar plating methods, in which melted agar is
poured onto the surface or utensil to be assessed. Upon hardening, the agar mold is placed in a
Petri dish and incubated. Angelotti and Foter
6 proposed this as a reference for assessing surface
contamination, and it is excellent for enumerating particulates containing viable microorganisms.
35 It
was used successfully to determine the survival of Clostridium sporogenes endospores on stainlesssteel surfaces.
83 Although effective as a research tool, the method does not lend itself to routine use
for food plant surfaces.
Sticky Film
The sticky film method of Thomas has been used with some success by Mossel et al.
82 The method
consists of pressing sticky film or tape against the surface to be examined and pressing the exposed
side on an agar plate. It was shown to be less effective than swabs in recovering bacteria from wooden
surfaces.
82 An adhesive tape method has been employed successfully to assess microorganisms on meat