By Prof. F. Marzullo – Division of Clinical Research – Istituti Ospedalieri – Modena

Studies have been carried out on behalf of CISP the Italian Vitreous Enamel Association on the bacteriological properties of surfaces of various materials, both native and specially treated, and stored in a refrigerator. The studies were intended to show

1) The level of spontaneous infection in contact with milk, broth or meat;
2) The level of infection provoked by contact with known bacterial cultures;
3) The ease of decontamination by physico-chemical agents

The following materials were examined:

1. Stainless Steel
2. Anodized aluminium
3. Native aluminium
4. Painted aluminium
5. Enamelled sheet iron
6. Painted sheet iron
7. ABS Polymer
8. Polystyrol

Materials and Methods
Nine types of plates were used (50x50x12.5 mm).
They were sterilized by UV-irradiation and placed in Petri dishes which had been sterilized by heating.

A) Spontaneous infection
This was followed by studying the development of the spontaneous bacterial flora present on meat, milk and broth. The bacterial load of the three materials was determined initially and after 24-48-72 and 144 hr.

B) Provoked infection
was followed by studying the development of colonies of known titre (i.e. number of organisms) of Staphylococcus pyogenes, Escherichia coli and Proteus mirabilis for 24, 48, 72 and 144 hr.

C) Ease of decontaminationThis was studied by following bacterial growth (at 24, 48, 72 and 144 hr.) on surfaces treated with soap and water; water and acetic acid; water and bicarbonate, applied with a brush or cloth.
A cloth or soft brush was preferred so as to avoid scratches which would prevent thorough cleaning and so favour the retention of bacteria.
This phenomenon was separately studied and we showed that even minimal abrasions led to this, particularly with surfaces of ABS, painted iron, polystyrol, painted aluminium and PTFE.
There was little effect with stainless steel or anodized aluminium and none on enamelled iron. In order to give a better picture of any changes, the foodstuffs tested (milk, broth and meat) were first held in an incubator at 37°C until a sufficiently high bacterial level was reached (900 x 107 c.c.). Fifteen tests were carried out for each investigation.

Virtually identical results were obtained with milk, broth and meat and they are therefore unified. Table 1 gives the mean bacterial levels found in the 15 tests.

SURFACEBacterial level (x 10-3)
0 ore24 ore48 ore72 ore144 ore
Stainless steel5.0303.0503.0903.0303.020
Anodized aluminium5.0703.9004.0103.9004.000
Native aluminium5.0703.0903.0904.0503.000
Painted aluminium5.0304.0003.0703.0502.900
Enamelled steel5.0702.0152.0002.0202.020
Painted iron5.0703.1003.0603.0603.060
ABS polymer5.0703.0253.0703.0903.025
N.B.: The figures are in thousands

Here too the results were virtually constant and those from the 15 tests are averaged in Tables 2, 3 and 4.

0 ore24 ore48 ore72 ore144 ore
Stainless steel200.00050.00040.00030.00020.000
Anodized aluminium200.00050.00050.00040.00020.000
Native aluminium200.00045.00045.00040.00035.000
Painted aluminium200.00030.00015.00015.0007.000
Enamelled steel200.00015.0005.0001.000650
Painted iron200.00020.00020.00010.00010.000
ABS polymer200.00030.00028.00020.00010.000
N.B.: The figures are in thousands
SURFACEESCHERICHIA COLI – 10-3 x no. cells at
0 ore24 ore48 ore72 ore144 ore
Stainless steel300.00075.00050.00030.00025.000
Anodized aluminium300.00068.00060.00018.00018.000
Native aluminium300.00070.00060.00050.00050.000
Painted aluminium300.00040.00020.00016.0009.000
Enamelled steel300.00018.00010.0005.0005.000
Painted iron300.00040.00040.00010.00010.000
ABS polymer300.00030.00030.00030.00018.000
N.B.: The figures are in thousands
SURFACEPROTEUS MIRABILIS – 10-3 x no. cells at
0 ore24 ore48 ore72 ore144 ore
Stainless steel400.000100.00090.00060.00050.000
Anodized aluminium400.00090.00070.00070.00070.000
Native aluminium400.00090.00060.00060.00050.000
Painted aluminium400.00050.00030.00015.0007.000
Enamelled steel400.00028.00014.00010.000850
Painted iron400.00060.00040.00030.00010.000
ABS polymer400.00070.00060.00060.00060.000
N.B.: The figures are in thousand

Here too the results were virtually constant whichever detergent was used (soap and water; water and acetic acid; water and bicarbonate). The results of the 15 tests under each condition are averaged in table 5.

SURFACEDECONTAMINATION – 10-3 x no. cells at
0 ore24 ore48 ore72 ore144 ore
Stainless steel900.0005.0005.0005.0004.000
Anodized aluminium900.0008.0007.0007.0006.000
Native aluminium900.0009.0009.0009.0009.000
Painted aluminium900.0005.0003.0003.0003.000
Enamelled steel900.0001.000700600600
Painted iron900.0007.0005.0005.0005.000
ABS polymer900.0005.0005.0004.0003.000
N.B.: The figures are in thousand

The following conclusions can be drawn from these results:

A) Spontaneus infection
On all the surfaces the bacterial level undergoes a fall which is apparent even at 24 hr. The greatest fall, which reached or exceeded 50% of the initial value, was on enamelled steel.

B) Provoked infection
No matter which organism was studied there was a rapid and significant inhibition of microbial growth. The greatest effect was on enamelled steel.

C) Decontamination
There was good decontamination at every surface no matter how infected or washed. Again this was most marked with enamelled steel.
It must be pointed out that the action of cold alone is sufficient to inhibit bacterial life independently of the material studied. However the constancy with which some surfaces show different degrees of bacterial inhibition compared with others leads to a consideration of a direct effect. It may readily be imagined that greater smoothness, greater compactness, a greater or less possibility of roughening or still worse of abrasions plays a role of first importance in decontamination. As far as spontaneous or provoked infection is concerned however it is necessary to invoke some chemical or physical factor which acts either directly on bacterial metabolism or by producing changes in the medium used (i.e. in the food or culture medium) and so lead to an inhibition of bacterial life.
Similar findings were made in a study of the mould flora which is often present in refrigerators and which is so difficult to eliminate once it becomes established. The degrees of inhibition which were found with bacterial cultures were closely followed with the moulds i.e.

1) Enamalled steelmaximum inhibition
2) Painted ironmedium inhibition
3) Painted aluminiummedium inhibition
4) ABS polymerno inhibition
5) Polystyrolno inhibition
6) Native aluminiumno inhibition
7) Stainless steelno inhibition
8) Anodized aluminiumno inhibition
9) PTFEno inhibition

The most significant part of this work is probably represented by these data taken together with those in the experimental part.

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